The goal of TFT is straightforward; build a team of units to win against the teams built by other players in an 8-player lobby. You roll and level up to upgrade your units and your team. The last player alive in the lobby wins. Beyond the basics, there are many subtle, lesser known, gameplay intricacies that a player can interact with to optimize their win rate. Here are a few small but meaningful actions that you can use to optimize how you play TFT.
When you get a carousel unit, you can actually sell it right before the next round starts. This means that you can go past an interest breakpoint before the next round's interest income is calculated, earning you 1 extra gold.
To "sell carousel", when you pick up a unit from the carousel, wait for the carousel round to end and the Little Legends are teleported back to their boards. This takes a couple seconds, and your new unit appears on your bench before the next round starts. As the carousel round ends, press
r to view your board, wait for your new unit to appear, hover your mouse over it, and press
e to sell it (you can't drag and drop units to sell during this time). If you press
e in time, you'll sell the unit and gain the gold before the next round's interest income is calculated.
Selling carousel not only let's you potentially earn more interest gold, but also keep a stronger board. Instead of selling important units just to go past an interest breakpoint, if you're right before a carousel round, you can keep those units and just sell your carousel unit.
You can have more units on your board than your max team size. To do so, right before a carousel round, make sure to have a full bench of units. When you pick a carousel unit, the new unit will end up on the board instead of the bench. As long as it stays on your board, it'll participate in fights. You'll also notice that when that happens, you have more units on your board than your max team size. If the extra unit activates more synergies, than those synergies will also actually be active in your fights. The cost to having this extra unit is that you can't reposition your team in this overflow state. The moment you free up a slot on your bench, the overflow state is gone and a unit on your board will immediately teleport to your bench.
This overflow state feels like an exploit but Riot has confirmed that it's an intentional gameplay mechanic. In fact, the in-game UI will also reflect this overflow state with the team size counter showing that you've exceed your max team size in red text (e.g. "5/4" if your max team size is 4).
Emptying your shop of bad units
A key game mechanic to understand with TFT is that the pool of units is shared between all players. That means that buying and holding units is a zero-sum game. For example, when you buy 3 Zeds, everyone else has a lower chance of finding Zeds because there are only 10 Zeds in the pool. This introduces interesting gameplay options like holding units your opponents need to make them weaker.
A lesser known unit pool mechanic to impact the luck of your opponents is buying and selling bad units in your shop. When a unit is in your shop, they're not in the unit pool, and your opponents won't be able to find them if they're rolling down. On the other hand, buying and then selling a unit puts that unit back in the pool, making it likelier for your opponents to find them if they're rolling down.
So the next time you find a bunch of Braums (poor Braum) in your shop, buy and sell them to gift your opponents a higher chance of finding Braums :)
Buying units while rolling down
The next time you're rolling down (refreshing your shop) to find important upgrades for your team, consider buying units of the same cost as the unit you're looking for and holding them on your bench. Because the pool of units is finite per game, and the chances of seeing units of a certain cost in your shop is fixed, holding units of the same cost as the unit you're looking for increases the chances that you find the unit you're looking for.
This is a small optimization like the previous tip but it can be meaningful and make the difference between a top 4 and a bottom 4 finish.
Rolling after everyone else
When you're running a comp that no one else in your lobby is running (you should scout to learn what comps players in your lobby are going for), one way to increase the chances of upgrading your comp is to roll for upgrades after everyone else has. Since you're the only one going for your comp, the other players won't be taking your units when they roll down. Because the pool of units is finite, when everyone else has taken units that you don't need out of the pool, you have a higher chance of finding your units.
To take full advantage of this strategy, you need to understand the common points in the game when other players start rolling (e.g. after every PvE round at stages 3-2, 4-1, 5-1 etc.). The most common time to roll is stage 4-1. You should also be scouting to make sure people are indeed rolling. Sometimes you'll find yourself in a greedy lobby where people start rolling later than usual to make more gold.
Running away to die later
The game only registers that you've died when the damage bolts flying to your Little Legend collide with it and reduce its health to 0. As such, if you die in the same round as other players, you can delay your death by having your Little Legend run away from the damage bolts the moment the bolts originate from your opponent's units and their Little Legend. Riot Mortdog confirmed that the speed of a damage bolt varies based on the distance between its source and your Little Legend to keep the travel time of all the bolts approximately constant. This speed is calculated at the time the bolt originates. As such, running away from the bolts the moment they appear will increase the travel time of the bolts, thus increasing the time they take to hit your Little Legend.
Sometimes, this is enough for you to die later than the other near-death players and earn a higher placing. This trick works best on maps with deep corners that your Little Legend can run to to be as far away from the damage bolts as possible.
Item mana bug
This is a bug that I don't endorse anyone to exploit. I'm just documenting it here for posterity's sake.
Every item with a tear component gives a unit 20 extra starting mana (Seraph's is the only item that gives 40). However, at the time you build the mana item on a unit, you have the opportunity to give the unit 20 extra starting mana. You can do so by placing a tear on the unit, wait until right before the next round starts, and then build the full mana item. Doing so will give your unit 40 extra starting mana (instead of the documented 20 extra mana), giving you a higher chance to win the round. Keep in mind that you can only do this once each time you build an item.
Here's an example: imagine you want to build Shojin's on Qiyana. You start out by placing your Tear on Qiyana, wait for just before the next round starts, and then combine your Sword with the Tear on Qiyana to create Shojin's. Normally, a Qiyana with Shojin's starts the round with an extra 20 mana. For this round, she'll start with 40.
Playing with and around Zephyr
Zephyr is an item that disables an enemy unit for 6 seconds. You can target an enemy unit by placing your unit with the Zephyr in a position that mirrors the enemy unit on the enemy's board. An opponent that's scouting will try to dodge your Zephyr because having a carry disabled for 6 seconds at the start of a fight can be devastating.
There's one trick to make it very hard for your opponent to dodge your Zephyr. To do so, you need to have an unattached Zephyr. Right before the round starts, you want to put it on a unit that is in the mirrored position on your board as the enemy unit that you're targetting. If you place the Zephyr fast enough, it'll activate and disable an enemy unit. The only way for your opponent to prevent this Zephyr from disabling their carry is if their carry is in a spot where there isn't a unit in the mirrored spot on your board that you can place your Zephyr on. You can't do this often since you can't detach Zephyrs from a unit without selling them so you should avoid placing a Zephyr on a high-value unit. Place it on a cheap and easily replaceable unit so you can ~abuse~ use this trick over multiple rounds.
On the flipside, if you're trying to dodge a Zephyr, and your opponent has a Zephyr attached to a unit and is itching to predict where you place your carry, you can make it incredibly unpredictable by doing what Souless does here: