Ship’s Mast

As the previous commenter said, to ‘play ship’s mast’ is to climb out of the passenger seat of a car with the belts out of both windows and to lie on the bonnet/hood holding the belts while travelling at speed. The film in question is called Deathproof however, not Grindhouse.

To tie belts to both front doors of a car. Then while speeding, climbing out through the passenger window, grabbing the belts and laying on the hood at high speeds. As seen in the movie Grindhouse.