Cluster training is one of my favorite tools to use in weight training. The old saying you can train heavy (intensity) or long (volume) but you can't train both is true, but cluster training starts to blur it very slightly.
A few examples. Workout 1. Lift your 5 rep max. Pause 8-12 seconds, rep out a few more reps, rest 8-12 seconds, rep out a few more. Your goal is to get 10 reps of your 5 rep max. It's a great stimulus to put on strength and size.
Sample workout 2. This is my favorite. Find your 3 rep max. Rest 10 seconds. Do a single. Rest 10 seconds. Do another single....etc. Try to get around 5 more reps. Quit when the rep becomes more of a grind instead of a steady positive concentric.
Try this when you're trying to bump up your Max or when you have a plateau in your training. I believe cluster training works so well it will be the norm for most programs in the future. It is a Central Nervous System (CNS) intensive workout, so I would only recommend this once a week at maximum.