The goal is to complete all the reps prior to the forced ones with good form until another good rep cannot be finished without help.
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.
The muscles could continue to move a lighter weight (see drop sets) or the same weight with some assistance.
The reps completed with help are called forced reps because, well…they’re forced. You can’t complete them yourself.
Forced reps can be used with many exercises and the help you get can come from a spotter or yourself on certain exercises. Obviously the exercise needs to be one that is easy to spot.
The most common example is probably on the bench press with the spotter screaming “it’s all you, bro!” while he’s doing most of the work.
Other common examples are barbell curls, leg extensions, triceps extensions and even squats (although risky).
If you’re using a single limb exercise such as concentration curls, you can use your free hand to force more reps after initial failure.
On pullups or chinups you could use a chair or bench to push off with your legs so that you can force more reps.
As with all intensity methods, forced reps are a way to push beyond normal failure but can be abused resulting in overtraining and injury.
It’s best to cycle their use and also be careful when combining with other intensity methods. Although the tendency is to use them on every set, a good time to add them is just on the last set of an exercise to push more muscular fatigue.
How many forced 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 form needs to be considered for your safety.
The general rule would be to add just a couple of forced reps, otherwise the spotter starts to do most of the work.