 When you see a snake gliding forward in its classic S shape, it's actually pushing off the ground, or substrate, wherever the snake is bent. Then there is rectilinear, and this one is one that Burmese pythons tend to use. Large, heavy-bodied snakes, such as large constrictors, will be very adept at using this, and that's because they're very strong muscles. So if you think of it kind of like a conveyor belt of sorts, where they're just moving straight, the body's straight.