From DJs link:
Quote 'You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.
A WIG craft, like a Hovercraft, rides on a cushion of air. The difference is that a Hovercraft generates a static air cushion (by blowing air into a bag) whereas a WIG craft has a free ride on a cushion that is created by its own forward movement. There are also lots of other advantages of WIG craft over Hovercraft, not the least being operational altitude.' Unquote
Oh dear, now they have said it's pressure too!!!! However their explanation makes perfect sense to me.