A wonderful, but not easy skill to help you work on your underwater pull phase is called the MFC. The momentary fist clenched as that is exactly what it is. A variation one the classic fist drill but the timing shift makes this so much more effective. Swimming with fists makes pulling harder, hoping we offset by using more of the forearm. In this version the effect of opening the hand mid pull suddenly makes the back of the stroke feel more more exaggerated and complete. The water gets heavier, you know it’s right.
When it comes to swimming, I feel the body is a remarkably adaptable piece of ‘machinery.’ If we encourage it into a position of reduced ability where it needs to compensate we then work harder to achieve previously similar levels of ability. The hand slips in its reduced state then slows as it retains it full size. The larger the object the slower it moves in water, the hand shape will suddenly feel very large and slow as the pressure builds from the increased drag around it.
By improving a specific movement having removed or reduced another area that previously was adding propulsion we have seen tremendous improvements achieved.
Here we are reducing the hand from being a paddle to something less than half the size in order to deliberately force the body to adapt your stroke and become more efficient. The shift from small to large helps activate the back of the stroke, reminds you to finish the stroke and helps you feel the water get heavier at the back of the stroke.
Two videos showing slightly different perspective
More details here and some different footage –TRI247