Place your hands shoulder width apart on the rail, next crouch down holding the rail to prepare yourself for the verticle spring.

The key to this move is to distribute your weight above your hands and not to lean either way too much. Also make sure that you keep your eyes on the rail/wall.

Once your hand leaves the rail you need to focus completly on where you're going to move your hand to to make sure that you're not left hanging on one arm.

Once your hand is back on the rail you need to see where your putting your feet.

Once you land on the other side you need to bend your legs slightly to absorb the shock, but not so much that you lose your balance and plumit to your death.