Relax, imagine that your muscles are simply bundles of elastic bands (not too far from the truth). When you stretch an elastic band and let go... It's uses it's stored "elastic energy" and the result is you hit your friend in the back of the head with said elastic band!
How does that concept relate to muscles? Simple, think about doing a bench press. You lower the bar to your chest and once it's an inch or two from touching your sternum you push it back up. Do you notice how when you increase the weight it becomes necessary (almost automatic) to "bounce" or "jerk" the bar back up in a very fast motion. What you're actually doing here is using the energy taken to stretch your muscle fibres and transferring this energy into the upwards part of the lift. This is cheating yourself! When you do this (and we have all done it) you are removing the intensity of the exercise and instead of using motor contraction in a controlled manner to move the bar upwards away from your chest you're simply using the elasticity of your muscle fibres to do it for you. This kind of lifting will only reduce the intensity of the exercise.
Next time you perform any exercise (in this example bench press) make a point of holding the bar at the low point for a couple of seconds, then start the upwards motion slowly in a controlled manner, no bouncing or jerking: if you can't stop and hold the bar at any point of the workout - you're cheating!
See how that feels.