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Dropping in is a skateboarding trick with which a skateboarder can start skating a half-pipe by dropping into it from the coping instead of starting from the bottom and pumping gradually for more speed. When a skateboarder drops in, they use their gravitational potential energy to gain initial velocity, allowing the skater to skate longer before exhausting themself and thus to take more runs.
When a skateboarder drops in, they stand on the deck of a half-pipe, put their back foot on the tail of the skateboard, and push it so that the back wheels just roll over the coping into a position like in a tail stall. They then put their front foot on the deck, which now hangs over the half-pipe, lean forward, and start skating the half-pipe.
Dropping in is an essential skill for anyone who wishes to skate a half-pipe efficiently, but it is also a very intimidating trick because it requires falling face down from a high place. It is very typical that when a skateboarder tries to drop in for the first time they fall on their back because they are too afraid to lean enough forward. 
- Youtube video Getting over the fear of dropping in
- About.com http://skateboard.about.com/od/tricktips/ss/HowToDropIn.htm