Sidesurf is the term given to inline skaters

Inline skating

Inline skating is a recreational sport practiced widely internationally. Inline skates typically have 2 to 5 polyurethane wheels, arranged in a single line. The in-line design allows for greater speed than roller skates and better maneuverability...

 who skate by placing their heels facing each other but separated by roughly shoulder width. In the sidesurf position, a straight line can be drawn through both heels and toes with the toes pointing away from each other. The motion is similar to being on a snow board however the toes are both turned outwards. Another term used for sidesurfing is "crabbing," as this is similar to the perpendicular style in which a crab

Crab

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...

 moves

Sidesurfing is normally done while skating down hills, just like snowboarding

Snowboarding

Snowboarding is a sport that involves descending a slope that is covered with snow on a snowboard attached to a rider's feet using a special boot set onto mounted binding. The development of snowboarding was inspired by skateboarding, sledding, surfing and skiing. It was developed in the U.S.A...

, because gravity naturally accelerates the skater. However, sidesurfers can learn to propel themselves forwards by rotating their hips from side to side while pushing their skates perpendicular to their direction of motion. Initially, sidesurfing is quite hard, and a lot of practice is required.