This is a legitimate method used in weight lifting, say you are doing hack squat leg presses with heavy weight. You get 4-5 reps and realize that it is too heavy to do more, but if you cheat and stray from doing strict reps, you can use your hands to help you do 3, 4, or even 5 more “cheat reps”.
In doing this, it helps to completely “shock” the muscle and you end up putting more stress on the muscle than if you have just stopped at 4 reps. The trick to this is not helping yourself out too much, just enough to finish the rep.
Also do this on your last set. Works great with bicep curls, everyone has probably cheated on accident in the beginning, but now you can explain how this is good. Use you’re shoulders / back muscles to help curl the weight up. This method was made famous by the all time great Arnold Schwarzenegger.
Be creative and try it on other exercises. You will be surprised how this can take your training to the next level. Just don’t abuse it and use it as a training technique you use occasionally to shock your muscles into growth!