you need to set an instance variable and every time it touches the trampoline add one to it.
on colision with trampoline and
platform is falling and ----- >
instance variable is 0 set vector y to -500
add 1 to instance variable
on colision with trampoline and
platform is falling and ----- >
instance variable is 1 set vector y to -700
add 1 to instance variable
on colision with trampoline and
platform is falling and ----- >
instance variable is 2 set vector y to -1000
add 1 to instance variable
and so on.
then to stop this
player is on floor ---> set instance variable to 0
hope i helped!