The power breakers hit the head ball with as much energy and accuracy as they can muster. However, unless you have acquired a great skill in controlling your cue ball while also creating high cue tip speed, you should probably not use this break. When playing on a bar box table, whether it is a a 7ft or an 8ft table, I have found the second ball break to be the most effective in making a ball, not scratching and controlling the cue ball.
I find the best way to approach this break is to place my cue ball on or just behind the break line and near the side rail. Use low outside English and aim to try and shoot the second ball into the back rail just before the corner pocket. The best way to do this is to imagine that the other balls are nonexistent.
Don't try hitting it with "Break Speed" as your cue ball might fly off the table, or you might lose the draw you are attempting to get off the object ball, and possibly scratch in the corner pocket. After hitting the second ball, the cue ball should draw into the side rail halfway between the second diamond and the lowest diamond and bounce back into the center of the rack. This will often stop the cue ball from flying around the table and finding a pocket to scratch in.
If this works the first time, that is, you didn't scratch and you made a ball, then remember exactly what you did and do the same thing next time you break. If it didn't work, then the next time you break make a small adjustment where you hit the second ball, or adjust using more or less draw and English as well at a different speed. Sometimes it is even necessary to break form the opposite side of the table.
Once you figure out what works best on that table just keep repeating the same things. It is a good idea to practice breaking a few times on the table your match will be on. If you are only given a chance to practice for a minute I suggest the best practice is finding the right break. The break is the most important shot in almost every pool game. Accuracy and control are the two most important considerations when breaking. I hope this helps in your search for excellence in pool.