It helps your rhythm to link the stern rudder stroke to the end of a forward stroke on the same side.
In calm conditions you can keep your forward hand low.
To get more power out of the stroke in dynamic conditions (e.g. in a following swell, strong tail-wind or surf), lift the forward hand to create a higher angle. This will position the working blade deeper in the water.
Protect your shoulder from injury by dropping your elbow on the working side to your hip.