An act of checking the opponent's hands or sword by shield on the right side, while thrusting forward with the sword under my shield arm. It is a recommended technique to do from binding below and on the right, and also after falling under the sword and shield.
It is unknown if a simple thrust with hands covered with the shield is also considered to be a stichlack. The text of play 1 hints that possibly yes, but it is then never mentioned in this context. For example the play 30 shows this action, it would work as a direct thrust forward with hands covered, but the term schiltslack is not mentioned there. Eve though this is exactly the action to which the play 1 refered to as a stichslack.