I gave this due thought when I was gonna build my trap…..to make it open what you have to do is:
#1 Make sure that the hose you run to the trap is hollow, and runs under the spring on the pedal(the large one wrapped in the black material) so that there is part of the hose sticking out from the pedal, into the area the spring will cover. And into the trap, so it pokes out into the area the doors sit. (by doing this, you can't take the main part of the trap off the base.
#2 When you attach the spring, make sure that the material the spring is attached firmly on the foot pedal and the base of the pedal, you want an air tight seal. Something like a glue gun should do the trick.
This way when you stomp on the foot pedal, the air under the foot pedal is sent out of the pedal via the tubing and into the trap, hopefully with enough force to open the trap doors. And when you step off the foot pedal, the air return to it's previous position under the foot switch. You would have to manually close the trap doors. Hope this helps