Another tuppence worth.

Edging is a skill. It is learned and can be honed and perfected. How much a sub can take without "disobeying" will depend on where they are in the training process. Expecting a totally green sub to start with hourly edging and keep it up for 12 hours is like expecting someone who has never even run to catch a bus to walk out their front door and run a 4.5 minute mile.

And frankly, sometimes the edge moves around. What has never tipped you over that edge, will sometimes just make you fall screaming into space.