Parkour/Basic Moves/Monkey Vault
The Monkey vault is an extremely common and useful vault that can be used for rails or walls up to chest height. As opposed to most vaults, where the legs swing around to one side or the other, in the monkey vault the legs are tucked up to the chest and the entire body passes through the parallel planted arms.
To execute a monkey vault build up a bit of speed while approaching the rail, and just before you reach it step your legs together -- maintaining your prior momentum -- and jump up and forwards, bringing your legs together up towards your chest. As you jump reach your arms forward and grab the rail, leaning forward to allow your feet to clear it. Once you have reached the other side you may choose to land on one leg to continue running, or if it is a significant drop execute a PK roll.