I hope ZedZded replies, but it looks like normal "longboard" technique.
When the mast foot is forward in the track, the sailor is not in the footstraps and is "railing" the Phantom on the centerboard to make max. upwind angle.
When the mast foot is near the back of the track, the sailor IS in the footstraps and is keeping the board pretty flat with the centerboard in the UP position. He is sailing on a reach or below and is in the fully
planing mode. Going for max speed without trying to stay upwind.
So...... normal longboard technique.... mast all the way forward, centerboard deployed board is being "railed" to go upwind on lift from the CB.
Reaching.... mast near the back, centerboard NOT deployed (reduces drag alot) and the sailor is back on the board in the footstraps.
Hope this helps,