Disconnecting speaker while power amp on - any dan

Something like this happened yesterday - managing to disconnect the left speaker (B&W 803S), I just missed to turn off the Rotel RB-1090. Nothing dramatic happened - there was not any “pop” sound from speaker, not the amp protection circuit was activated nor fuse burned... Probably because the preamp was turned off (or maybe on? - being upset at a moment I am not able to recall 100%..., but FOR SURE without any signal from the CD) Only my nerves suffered...

What is actually going on in such a situation, electronically speaking?

I can guess that the disconnected half (channel) of the amp “is seeing” a change in the level of resistance, unlike the other channel, still connected to the speaker. What a change is it? Is the disconnected channel “seeing” an ENDLESS resistance, or ZERO resistance?

IS THIS SITUATION HARMFULL FOR THE AMP CIRCUITS, even if no visible reaction of the amp (protection, fuses etc...)?

Electronic engeneers - any thoughts???

Thanx!!!