Yes, the force from the wing is indeed always pulling slightly backwards. Upwards and slightly backwards relative to the wind that is. Since the wind here is coming in slightly upwards along the slope of the beach instead of horizontally, the force from the wing is turned to clean upwards instead of slightly backwards.
Here is the link to a force diagram showing a gliding plane in another thread. Just change the glide velocity vector "V" in the picture with the opposite of the wind vector on the beach in the film.
Edit: Presumably, this exercise is difficult and dangerous as it takes place very close to the ground. However, the so called "ground effect" might add a bit of stability. The closer to the ground, the less induced drag a wing experiences; hence there is relatively speaking more lift the lower you go, or relatively less force to sweep you downwind.
Edit 2, on risks: After learning from another answer that the practice of hovering close to the beach is called "dune gooning", i googled it and it really seems like even practitioners deem the activity as "higher risk".