To press down on s.t.
The -an/pa--un imp. focuses on the body part of the agent that is pressing down on s.t. And it is for the benefit of the body part not for the affect on the object being pressed. The -un imp. focuses on the object that is being pressed down upon.
He pressed down the things in his suitcase so he could fit everything in.
Press your foot down on a bottle so that hopefully the pain will leave.