The dynamics of the catch are one of the more complicated parts of the rowing stroke. It can be quite hard to improve on the water because the rower has to work on two separate things - one being the control and placement of the blade, the other being the motion of the body mass relative to the boat.
The ergometer is a great place to work on being more efficient in your catch as you don't have the blade to consider and therefore can put more focus on the body motion. I particularly like the erg on rollers as we are moving our mass relative to the machine, similar to moving our body relative to the boat.
The motion on the recovery should be relaxed and fluid. This can be accomplished by using the lower abdominal muscles to roll the body out of bow with the knees soft (not locked) which in turn allows the body to move right out with the slide.
Roll out of bow
Nose and chin across the knees is a much better focus than hands across the knees. The head should be held level with the body relaxed. The bungee cord will pull you into the catch position so do not think about reach, instead think about compaction and getting shins to vertical.
See the photo at the top of this article for an example.
Use your hip flexors to pull your thighs to your chest, keeping weight off the foot stretcher as long as possible in order to reduce check. Imagine getting the feet out of the way while allowing the boat to come to you.
At the top of the slide, the goal is to get hold of the flywheel so that you can accelerate it. To do this, you need to engage a centrifical clutch so that you are moving faster than the wheel is spinning as you start the drive. (Imagine riding your bicycle - when you're going downhill, there comes a point where you can't spin fast enough to put any weight on the pedals so you shift to another gear).
You can accomplish this connection by reaching with your arms and engaging the lat muscles. The elbows will be low in a direct line with the core, just above your belly button and from here, you will pull horizontally with a level chain. We want to get tension on the chain before a lot of weight is put on the foot stretcher.
Elbows low, drive
This is the part that can always be improved and the long hours on the erg are a perfect place to practice. If you think leg drive before the catch, it's a guaranteed way to slow the boat down as your energy will be going against the foot stretcher without getting weight on the oar. While you might not pay for check on the erg, it will definitely cost you speed in the boat.