Cheat Reps are a set extending intensity method but they can also be used as an overload method. They are often abused for the wrong purpose of just moving more weight which decreases their benefit. When used as a set extending intensity method, the goal is to complete all the reps prior to the cheat reps with good form until another good rep cannot be finished. Even though your muscles cannot complete another rep with the weight you’re using because of in-set fatigue, that doesn’t mean they have been fully stimulated.
Muscles are generally weakest at their extremes (fully stretched or fully contracted position) and strongest in the mid-range position. So you may not be able to complete another rep simply because the muscle is fatigued and you can’t get the weight moving through the weakest point. The benefit of chest reps is you can use a little momentum (cheating) to help get the weight moving again through the sticking point so that you can continue to complete reps in the stronger position to push beyond normal failure. The reps completed using momentum are called cheat reps because you will “cheat” to get the weight moving and complete the rep.
When cheat reps are used as an overload method, the idea is to start with a weight that is just a little too heavy to complete any good reps with. In this case you will have to cheat on every rep to complete the set. Arnold Schwarzenegger was known to use this method during cheat curls. The purpose of the cheat reps is to overload the muscle by exposing it to heavier weights than it could normally move on its own. This overload will supposedly help to stimulate the most muscle fibers since all of them will be needed to try and move the weight. It takes experience to use cheat reps in this way since using too much momentum will simply move the workload to other muscle groups and keep the target muscle from doing most of the work, which is the goal. The risk for injury also increases since the tendency is to get the weight moving at all cost.
Cheat reps can be used with many exercises and the benefit is you don’t need a spotter since the momentum comes from yourself. Unfortunately, too many people use cheat reps all the time because of ego and using a weight that is too heavy to start with. We’ve all seen the guy bouncing the bar off his chest during bench press just to use more weight. If you’re trying to extend the set, you want to use a weight you can complete good reps without momentum. Once you can’t complete another good rep you will now use the least amount of momentum needed to keep the weight moving through the sticking point so you can complete another rep. The most common example is probably on barbell curls where you snap your hips forward to start the weight swinging upward. Another common example is barbell rows where you drop the body down to meet the bar as you row upward. Some risky exercises people use cheat reps on include bench press, squats and overhead presses. You have to be careful though as “bouncing” the weight to get it moving again can over-stress not only the muscles but the supporting structures like bones, ligaments, tendons. etc.
As with all intensity methods, cheat reps are a way to push beyond normal failure but can be overused resulting in overtraining and injury. It’s best to cycle their use and also be careful when combining with other intensity methods. A good time to add them is just on the last set of an exercise to push more muscular fatigue by extending the set. How many cheat reps you should complete is also up to you and how far you want to push past failure. Since you’re dealing with fatigue, your ability to maintain reasonable form needs to be considered for your safety. The general rule would be to add just a couple of cheat reps to push more complete stimulation of the muscle without resorting to questionable form.