The pressure comes from spring hinges.   The hinges maintain uniform pressure over a range of 3/4 inch which allows for lots of variation in wood thickness.   The spring tension on the hinges is adjustable.  The hinges are D&D Technologies TruClose® Gate Hinges, Model TCA1S3RC, available from Home Depot, Amazon, and others.  

The height of the fingers that press agains the wood is adjustable.  This is handy for re-saw because one finger can be set at bottom of fence and the other at top of fence, to keep the board vertical against the fence.

Details and demo in this video   https://youtu.be/PUPZONNRpGo