Why is the official guidance to entirely detach fall protective gear before moving from a mechanically elevated platform to a tower?
While taking the certification course for working at height, the official recommendation for moving from a cherrypicker / crane device to a tower was to completely detach your fall protective gear - such as the double lanyard - from the cherrypicker before reattaching the fall protective gear to the tower.
The unofficial advice was to operate normally and with common sense and make sure you've attached the fall protective gear before detaching and not just standing unprotected even for a split second.
What the instructor in the course was unable to explain, however, was why the official recommendation was to completely detach (and this was a course given verbally, so I unfortunately don't have a textual source to quote). He said something to the effect of "if you put the person who wrote that guideline in that exact scenario, you can be sure they'll make sure to attach themselves before detaching as well".
So... where is the source for this guideline - I'm not entirely sure what search terms to use, since I didn't take this course in English - and what's the reasoning behind it? This was given as the single exception to the rule to never, ever, ever be completed detached when working at height, so this piqued my interest.