Let me start off by saying that when use improperly momentum while weight liftining is the quickest way to getting a serious injury. So if you have little experience with lifting weights keep momentum out of your routines and stick with using strict form. You will get much better results with a lot lower change of injuring your self.
With that being said after years of being strongly against using any type of momentum during your lifts after reading some articles that natural bodybuilder Jim Crodova had written my views on this issue dramatically changed. I discovered that if and only if momentum is used in the proper situations it can be an added tool in your fitness tool box.
What do I mean? Well most the people you will see in the gym using momentum are resorting to it, because they are using weights that are far to heavy for their level of training. These people have 
the mentality of wanting to impress everybody in the gym with how much weight they are throwing around(improperly). If you take the time to analyze the movements they are doing you will probably notice that the muscle group that should be performing the exersices is lucky if its handling half the lifting load. The other half of the load or more is being lifted by supporting muscles and in many cases other muscles that shouldn’t even be involved in that particular lift.
When is momentum useful then? Well there are several situations when it can provide an added benefit for the lifter. One is when using it to overload and already pre-exausted muscle group. A good example of this is when performing bicep curls and on the last few reps you swing the weight a bit. However to do it properly you only want to swing the weight as little as possible just to get it past the sticking point(which is usually when your arm is just bellow being parallel to the ground. Once the weight has passed this point you should finish the lift entirely with the bicep and avoid arching your back to keep the muscle focus on the bicep.
Another good time to add in a bit of momentum is when your trying to save a supporting muscle group from reaching muscle failure before the primary group. A great example of this is when doing a front lat pull-down or any version of pull-down where your primary focus should be on the lats. In order to do this properly as you are preforming the eccentric portion of the lift(lowering the weight stack) you want to allow your body to sit up so that you
back returns to a 90 degree position. Then when you begin to preform the concentric phases(lifting the weight stack) you want to lean back slightly as you pull the bar back down to the front of your chest. The key here is the momentum is not used to lift the weight stack instead its used afterwords to allow the back not the biceps to control the eccentric portion of the lift. What you don’t want to do is use the momentum to help preform the lift and end almost having your back be parallel to the floor like the picture to the right.
So if you were like me an always saw momentum as a dangerous and bad addition to your lifting routine, maybe now you can look at it slightly different and see is value. But remember when all else fails just stick to weights you can handle and preforming exercise with proper form and you will be better off if your not sure how to properly utilize momentum.
