Cluster sets have been used for some time as a way to get in a higher volume of work at a higher relative percentage of your max. If you lift a higher percentage more times than normal, you'll get stronger. Easy, right?
For a cluster set you use a percent of your previously tested max (Cressey has oft suggsted 80%). One set will proceed as such: 2 reps...rack the bar and count down from 10...2 reps...rack and count...2 reps...rack and count...2 reps...rack and count...2 reps, done.
For the mathematically challenged (me) that's 10 total reps per set at a weight that you are, in theory, supposed to only be able to get 3 or 4 reps at. That's a ton of volume at such a heavy weight. In practice, this is damn near impossible. None of the guys I lift with have ever been able to perform a cluster like this at such a high percentage; we have always ended up taking the weight down to something much more civilian, like 70% in order to complete all 10 reps.
A more recent style of cluster training has appeared recently, "Every Minute On The Minute" or EMOTM. The idea is simple; set a timer for 10 minutes (or whatever time frame you choose) and hit your desired reps every time the clock hits a new minute. Depending on the lift, 1-3 reps is appropriate (anything longer starts to eat up your rest period).
You work up to the weight that you are going to use, and then just do the one set of EMOTM. It's a very easy way to get in a ton of volume. The longer rest period allows you to use a much higher percent of your max, which just increases the training effect of the rep scheme.
I did these the other day with 80% of my deadlift for 10 minutes. 365 for 10 singles felt like an appropriate (dare I say, easy) work load for that day, but I'm certain that I can use a heavier load on a different day when I'm looking for a different training effect.
This can be used with any of your big lifts: squats, cleans, bench, pullups, snatch, deadlift, press. I would, however, probably stay away from uni-lateral lifts as, again, that just eats up too much of the rest period. It's also sort of a silly idea to do Bulgarian Split squats at 90% of your max.
Give these a shot sometime soon and let me know what you think!
Have a great day, and go lift some heavy shit!