There are a lot of conditions that impose an AC modifier only as long as the condition lasts, as opposed to lasting beyond the end of the action that applies the modifier until your next turn.
Behind cover, blinded, cowering, helpless, kneeling, pinned, prone, squeezing through a space, and stunned are all conditions that apply a modifier to AC... but that modifier goes away as soon as that condition is removed.
If, on my turn, I take a free action to fall prone, I incur a +4 bonus to AC against ranged attacks, and a -4 penalty to AC against melee attacks. If I then stand up as a move action, this modifier goes away; I am no longer prone. The modifier does not last until my next round.
If I close my eyes, I am blind; I take a -2 penalty to AC and lose my Dex bonus. If I open my eyes, I am no longer blind, and the penalty goes away and my Dex bonus returns.
If I squeeze through a space on my turn, I take a -4 penalty to AC... but only while I am squeezing. If I get to the far side before my turn ends, the penalty is gone.
Some modifiers might change while it is not my turn. For example, if I am unconscious, I am likely helpless (-4 to AC) and prone (-4 to AC). If the cleric cures me, I will still be prone, but no longer helpless; my penalty is only -4, not -8.
If I am pinned by an opponent, I take a -4 to AC against melee attacks. If the rogue kills my opponent, I am no longer pinned, and the penalty no longer applies.
If I am behind cover (+4 to AC), and my opponent moves so that the cover is no longer between us, my bonus goes away.
In general the modifiers are phrased in two ways.
1. While [X], you gain a [Y] modifier to AC.
2. When you do [X], you gain a [Y] modifier to AC. This modifier lasts until the beginning of your next turn.
If the modifier is phrased as 1., the modifier lasts only as long as [X] is true. If it's phrased as 2., the duration of the modifier is independent of how long [X] lasts, because it is stated explicitly.
-Hyp.
