There should be no lock mechanism on your winch, if there is one then you are in trouble! the type of winch that uses a lock is free turning when the lock is released, that is the type of winch you should have on the boat trailer only.
I'm sure you have the correct type of winch because you mentioned the spring at the base of the crank handle. Yes, as Piper pointed out it has to do with the clutch mechanism. On the keel winch you should be able to crank the handle in either direction but it will always be in a controlled manner, the winch will never freewheel. It can only raise or lower the keel as fast as the handle can be turned.