It's actually fairly simple to have the trigger control an action with the pneumatics. What have you to do is declare some variable like handstate.
Then when your trigger is pressed changed the value of handstate to 1 or 0. Then when the trigger is pressed you know whether your hand is opened or closed and you can do an action based on it.
Code:
int handstate;
handstate = 0; //default position for whatever your using
if (p1_sw_trig == 1 && handstate == 0)
{
relay2_fwd = 1;
handstate = 1;
}
if (p1_sw_trig == 1 && handstate == 1)
{
relay2_rev = 1;
handstate = 0;
}