The key is to take your muscles to the point where you recruit these smaller assistance muscles. Learn why and how it demonstrates your intensity!
Temporary muscle failure is when your body is physically incapable of lifting a limb to a fully contracted position. Since that isn't good enough for me I take my muscle further to the point of not being able to move it more than 2 inches from it's resting position. This is achieved by doing partial reps once full range failure has been reached. Partial reps employ previously unused muscle motor units in order to assist the larger muscles in lifting the weight. The key is to take your muscles to the point where you recruit these smaller assistance muscles.

"Because I watched how his muscles were functioning on the previous rep and how he was wincing I deduced that he was holding back."

This brings us to the topic of intensity. Intensity correlates well with failure. Many times people believe they are going to failure, but it's a perceived exertion and not true failure. Case and point, yesterday my workout partner starts shaking his head yelling, "I can't do it" on our last set of leg extensions. Because I watched how his muscles were functioning on the previous rep and how he was wincing I deduced that he was holding back.

There was no doubt that he had a searing pain streaking through his legs as fast and as strong as lightening from the sky, but he needed to fight that pain in order to reach true muscle failure. Only by increasing the intensity in which he applied force to his leg extension reps would he be able to finish off the additional two reps the 3 partial reps, and then the masochistic 2 forced reps. Needless to say, with some coaxing and yelling from me, he reached this level or intensity in order to take his body to true failure.

This is the type of scenario that promotes the most growth from the entire muscle area you are working out. If you aren't taking your body to the point of pain and then passed you are short changing yourself on muscle hypertrophy and essentially wasting valuable gym time. It's not about who is prettiest or who picks up the most babes. To the hardcore bodybuilder the gym is about all out forcing your body to overcome its comfort levels of homeostasis. In short you can say, shut up, stop whining, do those reps until you can do no more, and grow like a monster.