Aim Pool

You can choose a target for your aim pool and gain tokens against them while actually shooting at other targets. Think of this as the situation where the archer is drawing a bead on the enemy leader when 3 mooks suddenly rush him. The archer drops all three mooks instinctively, without ever losing his focus on the leader.

If you choose a target and they remain still for their action, you gain 1 token, even if you do not take any actions yourself to aim at them (per the text, you gain this token at the start of your action, before you have even had a chance to aim).

You have to be able to see your opponent to have them as the target of your aim pool. If you lose sight of your aim pool target for the entirety of your action (i.e. you cannot see him at the start of your action, and do not get into a position where you can see him by the end of your action) then you lose your accumulated tokens (note that this makes “building tokens on that guy and firing at those guys” a little more risky, since “that guy” might move out of sight).

Who can you spend Aim Tokens against?

See Using Tokens for details.