I did a bit of messing around with this before.
When an AddAction is used, there are 4 things passed to the script:
_this select 0 is the object which has the action
_this select 1 is the object 'using' the action
_this select 2 is the number '0'
_this select 3 is a null value
If you try more (_this select 4 for example) you start getting Zero division errors.
I tried to figure out what the Zero and Null values represented, but I couldn't find anything. I tried all the settings for the object and the ones for the unit 'using' the action, including health, rank, init, skill, name, all of them. Also I tried speed, altitude, rating, units killed but non of these affect the Zero and Null values.
Also I tried using members of my team to use the action but this didn't tell me anything about these 2 values.