i either press the middle button tons that works for me. Or you can wait till it says PUSH! then press the middle button once an d the bar will go up and down. Then press it when you think the bar is highest and it will start bumping the opponent
When I see it full I go Na, na, na, na, na, na, Na! (This is tune indicating when the bar is full) and push right before the next "na" and it goes full. It works. *winks*