Experiences of Cycle Training

Pointless random rearward checks is bad advice.
...
Other than that maximum respect to your NS..

Trainees do sometimes worry about this - that they will hit something in front when they are looking behind. It's all about the dynamic road situation and adapting your rear/forward view to the circumstance. And adapting your speed.

It does take some people a while to get it and as Will hints, timing is crucial and comes with experience. The beauty of 'our' National Standard (as opposed to cycling proficiency) is that it isn't proscriptive. This means that cycle trainers communicate principles: instead of saying glance back every 5 seconds or hold a signal for 3 seconds, we teach people that a glance back will ensure the rider isn't surprised and will make a driver aware of the rider causeing the driver to give more space and pass at slower speeds or hang back till there's more room to pass. A glance can be really quick unless a stare is required to hold the driver at bay. We teach scanning ahead constantly and judging when to look back doing so when it prudent.

(In fact the only fixed-rule measurable guidance we teach is riding a** car doors width** away from parked cars and even that is dynamic when the road narrows and there is an oncoming driver)

I have noticed that some people do find it hard initially to extrapolate behaviour from principles since such a lot of other learning people are exposed to is so proscriptive.