Concept art of Tux sliding (Left), with example of tux sliding down a ramp. (Right)
If you are playing some of the newest nightly builds of ST, Tux is able to slide by holding down while moving. Speed can be built up by going down inclined terrain or by jumping.
I am pretty sure the latter method of building up speed by jumping repeatedly on land could be abused by speedrunners. I tested it, and Tux is slightly faster while sliding than walking
