This is a rather difficult
skateboarding grab, as it involves the removal of one foot from the board. This is quite tricky to do and requires a lot of practice.
Go off a jump and get alot of height. Grab the board in a mute grab then bend your knees up as if beginning a tweak. Before getting to the tweak stage, however, take one of your feet off the board and straighten it out down below you. When you are ready to land replace your foot and return to your normal position.
Note: This is a very time consuming trick. Get alot of air before trying it. It is also virtually impossible to add into a combo.