It is an indirect way of firing, according to Hoyer (see link below) invented by the older Puysegur and used first in 1644 during the siege of Gravelines.

In order to hit a target which cannot be fired at directly, a wall (if existing) behind the target is shot at in such an angle that the rebouncing shot hits the target. A bit like in a pool billiards game. As it is not very efficient, it should be used only if there are no other means.