We only need to open one gate at each end to get the boat through, which saves a lot of effort. Once in, the theory is that if the boat is kept close to one side, and the paddle on that side is opened first, the boat will be pinned by the flow rushing underneath. In theory, practice and theory are the same, but in practice they are not. However these locks mostly seem to have read the text book. The flow creates significant turbulence, but it seems to pin the boat as expected. So much so that in some locks, Clare nipped in to hang out washing from the machine while the lock was filling.