Trigger Guard is one of the four universal mechanics which require the use of Bullet Counters and it is the equivalent of a "defensive burst", functioning as a combo breaker.
While your character is in hitstun, press Trigger button plus Guard. If the input is successful, one Bullet Counter is used, a gunshot sound effects plays, and the gun cylinder near the Bullet Counters (see the HUD page) will rotate.
The character performing the Trigger Guard will exit hitstun and reset the opponent's combo counter. If the opponent attacks while Trigger Guard is active, they will suffer a blowback. Trigger Guard is active for up to 30 frames, as long as the Guard button is kept pressed. As of version 1.0, the player using Trigger Guard has an instant recovery, meaning that they can use the blowback to retaliate immediately (netting an impressive 24 frames advantage to the defender). This is being changed as of version 1.1, which will instead give the defender no advantage, thus resetting the game to neutral.
Countering a Trigger Guard
Trigger Guard adheres to the same rules as a normal guard: a standing Trigger Guard cannot block a low attack, while a crouching Trigger Guard cannot block a Mid attack. Therefore, mistiming the Trigger Guard or using the wrong stance will net the opponent a free reset.