It stops the length setting shifting under hand pressure mid-trim, which on a forty-position wheel is the difference between one length and a visible step.
A zoom wheel sets the guard length by moving the comb rather than by swapping it. That is fast, and it introduces a failure a bag of separate combs does not have: the wheel can move while you are holding the tool.
The finer the ladder, the worse it is — and both Brauns here publish a lock. The BT5525 gives 40 lengths in half-millimeter steps with a lock button on the wheel; the BT9545 gives 52 in quarter-millimeter steps with the same lock. On the finer of the two, two clicks knocked out of place is half a millimeter rather than a whole one, which makes the slip harder to see and not less costly.
The symptom is unmistakable and usually diagnosed as something else: one side of the face shorter than the other, on a trimmer that was set once. Before blaming your technique, check the wheel.
Where a tool has a lock, engage it after setting and before the first pass. Where it does not, check the wheel each time you change your grip.
It is also worth checking after transport. A trimmer that has been in a bag has been under pressure from every direction, which is a separate argument for a case.
Last reviewed August 22, 2026