This remind me of the traditional idea of the 'Dead Mans Handle' fitted to trains etc.
That is close to the answer, the origin of Walk Assist on pedelecs was before there was anything permitting it in EAPC law, but there was a law permitting pedestrian controlled vehicles. These usually took the form of powered four wheel barrow or small van with a motor switched by a tension switched handle. Pick up the handle and walk off and the barrow would then faithfully follow you along at walking pace. They were commonly used at railway stations for luggage or bulk mail.
Up to 400 kilos in weight no driving licence class applies, but beyond that there is a driving licence group and driving test to ensure safety. Our law for those dates from 1963, well before e-pedelec law and there is similar elsewhere. I believe Swiss company BikeTec who make the Flyer pedelec brand were the first to take advantange of this by adding walk assist to their Panasonic powered models in 2002. Of course to use this law on EAPCs the motor power had to be controlled at all times by tension from the person walking with the bike, automatically cutting power if released.
Since then and much more recently Start Assist has been permitted, hence the confusion of why the handle originally had to be switched when walking.
To complete this information, when a bicycle is propelled by its rider walking alongside and not scooting it on a pedal, a test case** has ruled that the bicycle is then luggage, so no pedelec or motor vehicle law regarding having the EAPC with you can apply then.
**Waller L J in the Court of Appeal, (Crank v Brooks [1980] RTR 441).
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