Need advice about one way collision

Here the code from platform tutorial from TDN-Torque Game Builder

// in air and hits platform with head
	if(%normalY == 1){
		%this.airborne = true;
		%this.setLinearVelocityX(0);
		%this.setConstantForceY(100);
		%this.setLinearVelocityY(%yVelocity);
		return;
	}

I want to detect one way collision but I don't know how to do it...could someone help me?