The glass walls are always a part of the game, but there are some rules for how they can be used. When receiving a ball, you can let it bounce up onto the glass wall after it has bounced on the ground, the ball is then still in play. You can also use your own glass wall to hit the ball back, it can be useful if you are in a tight situation. The ball may only bounce up onto the mesh when you receive it.
The ball is then in play if it first bounces on the ground and then up onto the mesh (this does not apply to service). If you hit the ball onto the mesh on your own side, you lose the ball.
The rule on your own side is that you may only utilize the glass wall. If a ball goes directly onto the opponent's mesh/glass, i.e. does not bounce on the ground first, then the ball is considered to be out.