Had my Westy over 4 years and never knew it did this. Saw a post some time ago on faceache that prompted me to see but at the time I did not get it to work. Tried again today and hey presto
There are 2 little catches, 1 on each side, spring was missing on one side of mine & needed persuading round into position.
The little round knob has to sit in the notch.