Holmes was brought to Vahalla to hit bombs. He is one of the longest players on tour. He has the ability to overpower a golf course. The Americans are hoping he will overpower the Europeans.
Holmes does a great job of getting his body behind the golf ball. He does not just slap at the ball with his hands, which provides very little power. Rather he drives his right shoulder into the ball. Watch during his downswing how his shoulders appear to drive down and through the ball. This is a very powerful move.
If you are looking for more power try to copy this move from Holmes. It will also help you create proper contact. You should hit the ball your irons while the club is still traveling towards the ground. A lot of players pull their shoulders up trying to get the ball in the air. This causes a lot of thin shots. Instead feel like you are driving your shoulder into your ball. That will help you get a great ball flight.