Commentary
This plays shows a possible in-tempo counterattack done from quinta custodia against besieger's halpschilt cut. The defender will catch the cut on his shield while thrusting from the other side at the same time. This action is very similar to play 20.
The last image is showing a possible way how the attacker can handle it. When the defender launches his thrust, the attacker beats it to the side with a schutzen, and immediatelly follows with a thrust. Such defence is only possible if the halpschilt was not commited and the defender was expecting something like it.
As quinta custodia can launch also halpschilt cut, the text also mentions a possibility of launching a different in-tempo counterattack - a separating strike.