Many exercise academics may criticise the lack of terminology used here for a concept that isn't new. Including sets on a non complimentary muscle group during sets of your core movement. Examples of this would be things like Benching (primary movement) and abdominal planks (micro set) in between or squats with wrist curls, chins paired with calf raises, overhead press with glute bridges etc etc. the list of possibilities is endless..
So the question is why do this?
This is where the benefits are plentiful. Predominately you can pack in more volume. You can increase the metabolic effect of the training. If you have a requirement to focus volume on a weak point like calves or grip strength or small muscle groups like rear delts. Doing this also promotes "blood shunting" around the body which can alone be great as a recovery technique.
How to do this?
Firstly you need to determine your objective. If you want to get brutally strong and am focussing your attention on just recovering for the next set as you are maxing out I suggest you avoid this. If however you are "punching the clock" in your training cycle and feel you want to add some additional volume then great. Select a movement that does not add additional volume to your primary movement. Bench then calf raise, squat then lying DB flyes, chins and front delt DB raises. The key thing to highlight here is this is different then doing antagonistic supersets eg bench and row, squat and leg curl, barbell bicep curl paired with tricep press downs. You do these supersets as a way of flushing a lot of blood into these primary body parts. The type of micro sets been suggested here are primarily to increase volume over the course of your training. For example if you do calf raises in between sets on 4 days a week it very quickly increases the volume over the course of the week, month and year
If there is anything to take away from this post it is that volume can lead to improvement provided you program this in appropriately. Small muscle groups paired with major movements can be a simple but highly effective way of bringing up weak points. Even if you're only focussed on strength and not mass one thing to remember is that a larger muscle has the capacity for greater strength expression. Take some time to think about what movements you might like to include in your training and give them a go. Some of my favourites are Neck flexion and extension with bands, single leg body weight calf raises and any abdominal exercise.
Now go lift hard, heavy and frequently!