Hyperspeed Walking (often abbreviated HSWing) is a glitch that allows Mario to build up speed while walking in steep slopes submerged in water. It is useful in many challenges including the A Button Challenge.
Steep slopes that normally could not be stood on can be stood on when they are submerged in water. If Mario walks uphill or perpendicular to uphill while standing on such a submerged steep slope, then his speed will decrease. There is no lower bound, so Mario can build up unlimited negative speed in this fashion as long as he can remain on the submerged slope.
Walls can keep Mario in place for small amounts of speed, but Mario will eventually break through them. Ceilings, which usually extend farther than walls and keep Mario in place rather than pushing him, are often more useful. Even better is Out of Bounds, which keeps Mario in the same place like ceilings do, but extend forever, allowing Mario to build up far more speed.
The speed gained by HSWing depends greatly on the normal.y value of the floor in question. On steeper slopes, Mario builds speed significantly faster, used in the faster strat for Watch for Rolling Rocks 0.5x A presses. However, the syncing speed is also greater on these slopes because Mario's de facto speed is less.