Watch his left hand. He holds on to the end of the string after he has weaved one ahead and tensions the previous cross while still holding onto the end of the string. He does not have to search for the end of the string when he goes to weave the next cross. Simple and saves you several seconds for every cross if you are used to searching for the end of the string.
I just stick the string that gets woven ahead about 6 inches through the frame , never have to search or hold.