Density training involves moving a lot of weight via volume and short rest periods. It invariably requires lifting lighter weight, but the total pounds moved in an hour are greater. Intensity is still high and failure is reached, but not with a handful of max-weight reps. The limited rest periods (think 15-30 seconds) ushers in the growth with less weight. Your joints and achy back will thank you.
For example, try doing occluded extensions alternated with heels-elevated goblet squats for 3 rounds, then remove the wraps and rest 3 minutes. Repeat a second or even third time, alternating exercises until you've done each one 3 times. Small weight, nasty pump and maximal results from minimal weight.
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