You don't hit and do damage until you've depleted the enemies defense, the blue bar, to empty. After that you start doing damage. The amount of your UAC that you apply to your attack is based on how full your red bar is. Swinging with a full red bar means that you have all 29 points of UAC behind your punch.
The idea is to keep swinging until you break the Blue bar and then start hitting the deer. Its blue bar will regenerate over time so the faster you can deal the damage the better.