It's a pixel surf so you would just hold down the A or D key into the wall as you walk onto it and just face forward. Once you're on the surf, you let go of W and you should fly along it. It's caused by the top of a brush being in line with a floor/entrance on the same wall (if that makes any sense - if not, get me to show you in game). Masque has tried fixing this in his maps by placing a 1x1x1 clip brush at the entrance to the pixel surf. This makes it very hard to get onto it.