Double Trigger Activation
Overview
Example of DTA in a Least Portals run.
Double Trigger Activation (also known as Double trigger_once Activation or DTA) is a glitch that allows us to activate OnStartTouch events of trigger_once trigger zones multiple times.
It's possible because trigger_once isn't killed instantly. Instead, it's killed on update which is executed 0.1 seconds (6 ticks) after first trigger touch. And since OnStartTouch event is activated as soon as you touch a trigger, nothing stops you from activating it again by walking away from the trigger and back into it within that 0.1 seconds.
Limitations
Because of alternate ticks, we're allowed to perform individual inputs only on even ticks. This means that the maximum number of trigger_once activations we can do in singleplayer with normal input method (keyboard) cannot be larger than 2 (it takes two ticks to leave the trigger, two ticks to enter it again, and then two ticks to leave it again, and at that point the trigger is killed so you cannot enter it the third time).
However, with unusual input methods (controller), it is possible to enter unique inputs on odd ticks, giving you an opportunity to activate a trigger more than twice. It's even easier in coop, where alternate ticks are disabled.
Thanks to that, you can touch trigger_once three times in coop and even four times in singleplayer but only if the first touch was made on even tick. TODO: explain why is that
Usage
This technique is mostly useful in Least Portals challenges, mainly for Laser Stairs in 0 portals (where DTA is used to drop the cube from the dropper second time, allowing it to roll to the player) and Laser Relays in 0 portals (to spawn two cubes).