If i remember, it has to be at a minimum certain distance from the poly directly below it. Not sure how much, possibly 100-200ish ingame length units (afaik, 1m=14-18px); there was somewhere a discussion about that.
To prevent parachutes you just need to have a poly (any kind) below that spawn point.
If the distance from the poly is correct, the players will spawn with a parachute.
Mind you, this was for older versions, and it is directly map related, has nothing to do with scripts.