I bring partial reps into play on movements where forced reps (a spotter is used to provide enough assistance for the trainer to be able to complete the rep) are impractical or potentially dangerous (for example, barbell rows or squats, respectively). The range of movements for which I use partial reps are most forms of rowing, all versions of shoulder laterals, hamstring curls and various types of squats.
This is how I employ the Partial Reps Principle with barbell rows: After the required number of warm-up sets, I load the bar to 375 pounds, with which I can normally do about eight full reps. When I reach failure (I can no longer complete a full rep), I attempt as full a movement as possible by pulling the bar as high as I can toward my abdomen. I may manage a three-quarter movement, or it may be a half movement. These less-than-full reps are called partial reps; as with forced reps, they provide a means to go past the normal point of muscular failure. I'll do 2-3 partial reps, which successively become shorter in their range of motion as my strength diminishes. After three partials, I'd be down to maybe a one-quarter movement, so working past that point would be unproductive.
To guarantee maximum intensity, I do forced reps or partial reps at the end of every main set. If you do multiple main sets, you can do partial reps for one set only. If you did them set after set, you'd over train and burn out and perhaps become injured.