This drill is one we do fairly regularly as part of our warmup so I wanted to quickly go through it and differentiate between all three to explain what their individual purposes are. The overarching purpose of the drill itself is to teach the rowers how to distinguish between the functions of the outside arm and the inside arm.
How they’re done is self-explanatory … you row with just your inside arm, just your outside arm, and then with a wide grip. If you’re not sure what constitutes a wide grip, you can go one full fist over from where your inside hand is normally positioned (so instead of two fists between your hands now there’s three) or you can just put your inside hand on the far end of the handle either where it meets the shaft of the oar or just past it.
This is the version with the most variety in terms of what it aims to work on. One of the things we work on with it is catch placement. Rowing with just the inside arm puts the emphasis on placing the blade and finding an immediate grip on the water and takes the focus away from muscling (aka forcing) it in, which some rowers have a tendency to do. This usually happens because they’re lifting with their outside hands to get the blade in instead of unweighting the handle and/or they’re carrying a lot of tension in their shoulders.
Another thing rowing with the inside arm works on is keeping the inside shoulder relaxed and loose. It’s common for rowers who are switching sides to pull too hard with what used to be their outside arm, which creates a lot of tension in the upper body. (Rowers on their normal sides can do this too.) Your inside arm doesn’t have the leverage to yank the oar through the water though so this allows you to focus on keeping the inside shoulder loose and the body controlled as the wheels change direction.
When you’re first learning to row a lot of emphasis is put on learning which hand squares and feathers the blade and which one actually draws it through the water. The inside arm is the one doing the feathering and squaring so if you’re coxing a younger crew, this should be a point of emphasis throughout the drill to get them used to rotating the handle with just their inside hand. Once they’ve got a good understanding of this, you can have them add the outside hand but keep it flat (i.e. just their palms resting on the handle) throughout the recovery so they can focus on keeping their outside wrist flat while the inside one rotates. It’s also the one that guides the handle during the recovery so if set is an issue, this is another opportunity to work on keeping it level without the influence of the outside hand.
Whenever you talk about hang or suspension, this is the arm that’s doing it. Rowing with just the outside arm emphasizes this and gets the rowers to use their body weight to hang off the handle in order to move the boat since, similarly to rowing with just your inside arm, you’re in a weaker position to get the blade through the water when you’re trying to do it with just one hand.
This drill, because it slows the drive down, also gives you plenty of time to focus on your body position at the catch and throughout the drive. When you’re coxing them through it you should be emphasizing what should be happening with their body so that their weight is being used efficiently. (Check out the “what to look for” section in the post on suspension that I linked up above for more on this.)
In terms of bladework, the outside hand is the one applying vertical force to the handle so handle/blade height is a point of emphasis on the recovery, as is blade depth on the drive. If you see the blades going deep on the drive you’ll want to point that out and remind them to draw through horizontally with the outside arm and feel the connection in their lats, not their shoulder.
For the most part this is essentially the same as rowing with just the outside arm but with better balance since you’ve got both hands controlling the oar now. Similarly to the previous drill, it puts the focus on suspending your body weight off the handle while keeping the outside shoulder just slightly higher than the inside.
This is a good drill to do to work on rotating around the pin and keeping the outside shoulder up on the recovery. If you visualize a line between your shoulders, it should be parallel to the oar handle when you’re at the catch, which is a good reminder if you have someone in your boat who has a tendency to lunge.
Here’s a good video from USRowing that shows all three drills as they’re coached by Mike Teti (with his Cal 8+) and Kris Korzeniowski (with the USA 8+ and 4-).