Momentum is "mass times velocity." It's the same in weight lifting as in physics. If you're lifting your mass quickly, you will certainly be able to sling a heavier weight. But it is momentum that is providing the power for the lift, not muscle strength. You're cheating a whole bunch of your muscle cells of their full development.
If, when performing a dumbbell curl, you jerk the weight up quickly, you will need to contract the cells in the bottom part of your biceps muscle. But the rest of the biceps get to coast through on momentum -- at least until you have to slam on the brakes at the end of the movement. A whole section of the muscle then gets cheated and remains underdeveloped.
But slamming on the brakes does not simply put an end to the momentum you've generated. That energy has to go somewhere. Jerking the weights is dangerous because of the strain on muscles, tendons, and joints when trying to stop a weight that is traveling through space on momentum you initiated. It is therefore much more effective, as well as much safer, to slow the weight down, force as many cells as possible to contract, and minimize the risk of injury to surrounding tissues.
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