I see it as something like this:
http://www.qldzoo.com/snakes-move/"how do snakes climb or move in tight spaces? Climbing and movement in small tight spaces is achieved using a ‘concertina’ movement.
concertina-motion-snakeTo move in this way, the snake will extend the front part of its body along the vertical surface, gripping the surface with its ventral or belly scales. A tight grip is ensured by the snake bunching its body into tight curves while it pulls the back part of its body up to its grip point. The snake will then spring forward again to find a new place to grip with its body and scales."
Nicely illustrated here:
http://animals.howstuffworks.com/snakes/snake3.htm