The ideal way to break a 9-ball rack is to go strong, and hard. Speed is important, however, keep in mind that it is equally important to have total control of the cue ball. If the cue ball ends up in a pocket or off-table or in an awkward position preventing any good shots, it would put you in a bad position. Even if you manage to pot 4 balls with your breaking, it will not count as a good game as long as you do not have control over the cue ball for the next shot.
Be in control
There are a lot of techniques that you can use to add extra speed while breaking the rack. However, it is important that you master your control first and then try those techniques. Keep your body still, and move only your forearm. Once you have control, you can try pivoting your hips or stepping into the shot to put that extra speed into your breaking shot. You can try this exercise for getting maximum rack-break speed. Position the 1-ball on the foot spot, and the cue ball in a position ideal for breaking the main/head string. Position yourself to take the break shot, and do a stop shot on the 1-ball. Take the shot as hard as possible while making sure that the stop shot is a winner. Keep practicing this shot, softer at first, and then harder.
Once you have mastered your breaking speed, you can try this exercise for control. Arrange a rack of balls behind the 1-ball. Do a stop shot on the 1-ball. The idea is to get a nice scatter, a ball or two in the pocket, and an ideal positioning for your next shot. Keep practicing, and after some time, you will be able to find your speed, and control.
Points to note
Following are some points to note while breaking.
The ideal position to break is the one nearer to the side rail of the pool table.
Make sure to keep the cue stick as level as possible.
Make sure that the stroke is hard and firm and controlled.
A full 1-ball hit is ideal.
Try to get the cue ball in the middle.
Finesse breaks are ideal for 1-pocket and straight pool, however, when it comes to a 9-ball, go with a hard and strong break.