The overflow channel takes excess water from the top pound (above the lock) directly to the bottom pound when the gate is closed, bypassing the lock. This is necessary to ensure the water level above the lock doesn't rise to an extent that would make it difficult to open the top gate manually. Next to the walker is the mechanism which operates the ground paddle
Link , i.e. the sluice gate which allows water from the top pound to be fed into the lock through a culvert next to the lock gate. The corresponding mechanism on the bottom gate is usually a gate paddle
Link , i.e. a flap in the bottom gate below the water line of the bottom pound which can be operated from the top of the gate.
With thanks to Roger Kidd
Link for helping me understand how these locks work.