Whatever that is...
Tickles your nutsack.
It’s actually quite a little development. Basically, it generates force feedback that gives the user the impression that a rigid surface like a trackpad actually has “give” and provides tactile sensation - you can even feel the “click” made by pressing on the trackpad.
They use the same tech in the watch to provide subtle taps on the wrist as part of its notifications.
A small step perhaps to advancing toward true “holodeck” functionality.