These methods should be used to mix-up your workout so that you can grow. Don't use these methods every time you train or you will end up overtraining.
Stripping (Requires a spotting partner)
Example: While doing a 225 pound bench press you are able to get 8 good reps out. After your eighth rep your spotting partner takes 20 to 40 lbs off of the bar and you continue your reps until you can't go any further. Your spotting partner takes off another 20 to 40 lbs and you continue until failure.
This is a really intense way of lifting and you shouldn't do more than 1 to 2 sets using this method during a workout session.
Negative Reps (Requires a spotting partner)
Load the bar using more weight than you would use normally. Lower the weight under your own power and have your spotting partner help you lift the weight to the starting position. A person has more strength in the negative portion (lowering) of an exercise. Do a few reps and your muscles will start to burn from the lactic acid. Rack the weight.
Don't abuse this exercise because it can lead to over training. This is a really intense way of lifting and you shouldn't do more than 1 to 2 sets using this method during a workout session.
Partial Reps or Burns (Doesn't require a spotter)
You're in the middle of an exercise and you get to the last rep and you can't go any further. Using partial reps you continue to bring up the weight as far as you can (a few inches) and then lower it. Get out 5 or 6 more reps if you can. This will work the muscle until it burns. This burn means you have stimulated many of your muscle fibers.