One of the advantages of being off the water and on the erg during the winter is that you have an opportunity to practice the rowing motion on a totally stable platform. During this time, it is easy to row with level hands and a level chain. Time on the erg thus allows you to build the muscle memory for a level stroke if you can take that muscle memory with you onto the water.
I can almost guarantee that when we get back to coaching this spring, we'll see lots of rowers digging deep during the drive. There can be multiple reasons for this but at its most basic, the hands are simply too high; too much effort is being made to physically put the blade in the water at the catch.
On the erg, there's no need to lift or lower the handle - we recommend keeping the handle level throughout the stroke, and we want you to do basically the same thing on the water. If you were to let go of the oar handle, gravity would put the blade in the water and it will float at the right depth, just as they're designed to do. So instead of putting a lot of emphasis on lifting the oar handle, try to be lighter with less upward movement. (The timing of the catch is where it gets complicated. The catch needs to be the last part of the recovery, not the first part of the drive.)
At the finish, you can be just as subtle. It takes almost no effort to get the blade out of the water. If you do a lot of square blade rowing however, you have to tap down enough to get the entire blade out of the water. To keep the bottom edge of the blade from getting caught, you'll also have to pull down before you get to the finish, instead of pulling horizontally through. That's why we dislike square blade rowing - too much downward motion and it tends to shorten the finish. When you feather, as long as you get the top edge of the blade out of the water, you won't get stuck. The motion of the hands on the erg, with minimal up and down motion is closer to what you want to be on the water.
Keeping the hands more horizontal will help keep the boat more stable. If the boat is more stable, you'll be in a better position to apply more pressure on the drive. In turn that will make the boat go faster, which is what it's all about!