When you are breaking, it's most important to hit the center of the cue ball. This is one key component of generating speed and action on the rack. If you're hitting with too much power, you will have inaccuracies and hit the cue ball off-center. This will prevent the cue from transferring all the energy to the cue ball. The cue ball will not only have less energy but also deflect off the line, due to the applied side spin. You won't hit the first ball on the rack dead-on. This will once again fail to transfer 100% of the energy to the rack. Therefore, a less powerful attempt can make you more accurate, and at the same time, generate more energy at the rack.
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Cheers,
Shari