This is not a specific variable that makes the barrel explode, he just called "boom" a variable, and used it to trigger a mouvement to the barrel so it crushes on the floor and blows up. No master list of premade variables.
I know it is possible, but that is why I noted this as a possible improvement, because the solution to a multiple condition event is currently complicated in comparison to a hypothetical native multiconditional trigger that would allow for example a "on enter" + "if variable [x] has [y] value".
You can also put an object under the ground and the trigger makes it move fast and collide with the barrel before getting back to is position.
The good point is: you would make the entire thing work without visibly making the barrel jump and crash itself.
But it's still a trick that require an...
I've been messing around with the editor for two days and it's purely awesome, still there are some tools or functions that would make editing a lot easier and faster. A list:
A Parent/children system so we can easily assemble objects together and transform only the parent one.