Commentary
This play is connected to play 3 and play 5, because it shows different branch of the same action. It starts by besieging prima custodia with a krucke, while the defender is now not taking the bind - which means that he is defending only with the shield, not with the sword.
Because krucke is a draw cut, it is not stopped by the shield. When attacker hits the shield, he continues slicing across the shield until he gets the tip of his sword above the shield. Then he is in a great position fo thrusting - the better that he already was very close (because of short reach of krucke, he needed do get close for the attack). Even if the defender tries to step back, he still won't be able to get out of reach of the attacker's thrust.