What is a cheat rep? It’s when you lift more weight or do more reps than you can normally with strict form. There’s two different ways you can do it:
Extended cheat set: This is when you do a set with weight that you normally do with strict form and when you get to the point when you would normally stop and rest you keep doing reps using momentum or help from a partner
Heavy cheat set: This is when you lift more weight than you normally can through the whole set and loosen up form to do so
What are the reasons to do cheat reps? There are a few things to consider and that is how long you have been training, past injuries, and lastly the exercise you’re selecting.
The key to learning any new skill is learn the basics first, and yes weight lifting is a skill. You want to learn absolutely perfect technique and repeat that thousands of times to get that movement pattern engraved in your brain. This way it becomes how you do it all the time without you even thinking about it. Then once you become a “master” this is when you can play around with it and change it and see what works for you. I once read a quote by Eddie Cohen a powerlifting legend back in the day say “You have to learn the rules before you break them” and I am a big believer in that philosophy.
Also physiologically there is good reasoning to stay away from cheat reps at the beginning stages of a resistance training program. At the start of a training program there is a large potential for growth. It is so much easier for someone that has never lifted weights before to build 5 pounds of muscle then someone that's been lifting for 10+ years. When you are deep in training your muscles have gotten used to training so you have to stimulate your muscles in different ways and go past that failure point to elicit a response.
Another thing to consider before doing cheat sets or reps is your injury history. Cheat reps can put ligaments and tendons at risk due to you going past that failure point of the muscles. If you’ve already had problems in a particular joint I would not recommend you do any kind of cheating on a exercise that works out a muscle that connects to that joint.
Last and most important factor is the exercise selection. There are better exercises to cheat with than others for safety reasons. You never want to put a part of your body in a compromised position to strengthen another. A general rule of thumb is you want to to avoid bouncing weight to create momentum. Examples of this are things such as bouncing a deadlift off the ground, bouncing a bench press off your chest, or bouncing out of the bottom of a calf raise. The best way to cheat reps is if you have a gym buddy or trainer help you lift the weight slightly. If you don’t have a workout partner then the next best option is using momentum but making sure to not put your lower back at rick when doing so.
Some optimal exercises to do cheat reps with. Here are some examples:
Rear Delt Raise
Lateral Delt Raise
Bicep Curl
Tricep Pushdown
Single-arm DB Row
Lat Pulldown
Overhead Press (switching it to a Push Press)
What I believe is the best way to incorporate cheat sets is a supplement to regular training. Similarly to other more advanced training modalities such as drop sets, compound sets, blood restriction training, etc. I am more of a fan of doing extended cheat sets because of the inherent goal of doing cheat reps which is hypertrophy and volume is key here. A heavy cheat set you do the same amount of volume as you would regularly do just heavier weight which may be better for strength gains however I couldn’t find any research on that.
However that's not how most people use them because a lot of people do cheat sets literally all the time because they don’t know how to lift weights properly. People would rather be meat heads and fill their ego by lifting heavier weight than they can handle instead of formulating an intelligent training program. Jeff Cavaliere from Athlean-X always says in his videos “Putting the science back in strength” and I am a big believer in that. Be careful of the gym bro science that you hear out there and research topics that people say to do to make sure there is research to support what they are saying.