Jeff Liebermann has brought this to us :
Using an air compressor, I was cleaning out the dust from a
consignment of used ATX power supplies, when one of my friends pointed
out that I was killing the fan by spinning the fan backwards. What he
does it use a stick or toothpick to prevent the fan from rotating.
This seemed odd to me as I've been using compressed air to clean fans
by spinning the rotor in both directions and usually faster than rated
RPM for maybe 25+ years without incident. Google found some articles
proclaiming that spinning the rotor with compressed air was a bad
idea. However, I couldn't find anything describing any failure modes.
So, is using compressed air to clean fans really a bad idea?
Is it bad to spin a cooling fan backwards?
What breaks or fails?

Well I was told that so far back that I can't remember (1960s) I think,
Long before PCs, mainframes had lots of fans.
Although I never saw a failure I always thought it was because you
could grossly overspeed the fan if you were stupid enough to put too
much air in. B-)

--
John G Sydney.