In the picture above are two gates. (numbered 1 and 3)  The ball rolls from the right and goes left under gate 3 first.  This gate is really a switch activator.  When the ball pushes the gate wire that hangs below the flat metal plate, it pulls on a switch underneath the playfield (number 2 in the picture)  This switch may then activate some sort of sound or score.

The ball can move both ways through this gate.  The chance that a ball will go from left to right through gate 3 is pretty slim. Gate number 1 will prevent that from happening. It is a one-way gate, the ball can only go through it from the right to left.  Its sole purpose is to prevent the ball from going back down.  The idea is to keep the ball in that area so it rolls down into the pop bumpers, the 3 big white circular parts on the playfield.