Approach your obstacle, turn vaults are mainly displayed on railings and walls about chest or abdomen high. Place one hand under the railing and the other on top and hop over. The hand under the rail should be your dominant hand which is going to be your center of gravity in this Vault.
Whichever hand is under the rail is the way your going to turn. When turning the hand under the rail should be on top and your other hand should meet the rail at one point. When your on the other side you should be able to plant your feet on a wall or maybe a bottom railing.
Turn Vaults are mainly used to help you get ready for a big drop or brace for impact instead of just going directly for the jump.