This is where you grab your board on the heel side with your front hand.
Much like any other grab, this is just about getting enough air to grab and still be able to let go and land. Make sure when you're ollieing that you stay low, this will help you when trying to get the grab in. In videos you might see pros grabbing this then kicking their feet our more infront of them, which is nice style. You do need big air for that. Good Luck!