Ansetzen is the act of setting-on with the point to the opponent.
For example, if the opponent lifts their blade to their right shoulder, thrust in with ansetzen, placing the point on the opponent and covering that quarter with your sword (ie. thrust into left ochs)
If the thrust had been performed as the cut descended, and was in opposition, it would typically be characterised as Absetzen instead.
This can be seen in the Thrusting section in Application 1 & 2.