The owners manual says to engage the OD without pressing the clutch.
There is a low-speed limit on the thing, but you wouldn't want the OD at
that low speed anyway. In the last iteration of the OD, it operated only
on 4th. Most recent issue of MGB Driver has an article by John Twist
that covers your questions and more.
On Tue, 7 Apr 1998 14:40:55 +0100 (BST) Carl Mole <C.Mole@tees.ac.uk>
>What is the correct operation of the over driveunit.
>ie. what speed should the overdrive be enabled and disabled
>should the clutch be depressed on flicking the switch etc