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^^Bead stays locked in place, in theory.
It does, you can tell when you pumped up a normal tyres + tube on a tubeless rims that created an almighty crack sound as the tyres bead pop into the rims wall.
Even when fully deflated, you have to really push the bead off the wall to gain access, again creating a loud crack.
Actually thinking about it, you'd know it's a decent tubeless rims when it crack like that.
edscoble-
the crack does not always happen even with good tubeless tyres on good tubeless rims. IRC tyres which remain firmly locked in place on many rims with no air in the tyre do not ping and crack when locking in place. It can be a silent affair. Schwable tyres on the other hand do this with noise.
I have only unmounted a tyre in a race when something put a 20 to 30mm gash in a tubeless tyre in a lotus league race and I tried to continue racing. The tyre unseated and I had to stop. It take real effort which is a good thing.
cycleclinic
^^Bead stays locked in place, in theory - I'll need others to chime in here as I've not experienced a catastrophic puncture (i.e. one that is noticeable) and can't therefore speak from personal experience of what happens.
But- going with what I understand, the beads on either side stay locked in, you punch the plug through the hole from the outside, reinflate.
Think car and/or motorcycle tyre rather than bike tyre.
EDIT what Sumo said.