My training partner likes to say that past-failure techniques such as dropsets, cheat reps, partial reps, and even forced reps really only work because they give you a second chance at a high intensity set. Now this might sound like a bit of a philosophical viewpoint, but it's actually an ultra realistic one. Ultimately, intensity is the only deciding factor in growth and rate of growth. Yes, isolation plays a part as well, but your intensity level directly impacts how completely you exhaust each muscle, how often you train each muscle, it even determines the volume you use during your training.
As you approach failure, several factors may determine which technique, if any, you choose to use. Here are the most common past-failure techniques, along with a few guidelines and contraindications for each. Remember, use them one at a time!