With normal entities that have an animation set such as characters or switches and some that dont - activation is required for the entity to react to its environment.
In the case of decals you would need a special script to activate it using the switch - running or calling the decal only following the player throwing the switch. Under normal circumstances a decal will loop immediately the player enters the area and is always active - decals are not normally still - but play their animation frames continuously following game start and whenever the player is within a proximity hard coded I believe into the engine and called its script i.e. rundecal. The active range I think is dictated by the engine as supposedly with other AI entity activation/deactivation distances though theres some doubt as to whether or not that is faultless. (the always active flag - on/off)
I am not sure what you want to do can be done - as there is little control or flexibility when it comes to decals.
However it may well be possible to workaround with some thnking and script writing if any default engine decal inflexibilities can be over ridden.
Personally I do not have the time to find out now - it may be easy or difficult and time consuming to do I dont exactly know. My guess is it may not be easy to do without some trickery involved as a workaround though I may be wrong.
Not sure if you will find a script already around here to do it or anyone can advise more precisley.
Seaching may help or someone surely will reply further if you wait a little longer
