You'll never stop the blade spinning immediately, even if the power
does go off.

It's technically possible to stop the blade almost immediately with a
brake. But the momentum of the spinning blade has to go somewhere and
for a handheld saw that would simply jerk it out of your hands.

I don't believe Ben's statement "I have been on jobs where I have had
no alternative but to use a circular saw ". Using a circular saw
because that's all you brought with you is not a safe condition, or a
valid condition to claim "no alternative". Even if you wanted to use a
powered saw, a reciprocating saw (jigsaw or sabre saw) is much safer
than a hand-held circular and all its reaction forces.

I work on timber-framing projects with untrained apprentices running
around. No-one goes up onto a frame with a circular saw, nor do they
ever need to.