Sperm usually swim in a 3D shimmy: a spiral wave travels down the whippy flagellum and rotates its head in a circle around its long axis. That "bulk swimming" is fine most of the time, but it isn't a great option when a sperm cell gets close to a surface. That's when they switch to "slither" mode.
David Sinton and his colleagues at the University of Toronto have found that when bull or human sperm are within one micron of a solid surface, they use a 2-dimensional swimming pattern reminiscent of a snake's side-to-side motion. Instead of beating the flagellum in a spiral, sperm beat it in a flat sine wave.
Sperm don't stay in slither mode for long — bull sperm slithered for less than a second at a time — but when human sperm slither they move 50% faster and straighter than they do swimming in 3D. The researchers think slither swimming may let sperm move through the narrow passageways of a cervix and the tight and twisty entrance to the Fallopian tube more easily.