That's coding.
That doesn't mean there isn't a way to fake it tho.
Look through the info_player_start's outputs.
I know there is something there which has to do with being hit by a stunstick, just set that specific output and trigger it to hurt the player 10 hitpoints.
It should work, if you find it.
It's probably the same output used to knock the player down in the beginning of HL2, I've used it myself, but I can't remember if I did it with the HEV or without it.
The name of the output should be rather obvious, just take a look.
info_player_start.