Jeff Liebermann wrote:

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?

For Pc fans, it's harmless to clean them with compressed air, unless
you're trying to overspeed them by angling the air stream just right,
turning the fans into turbines.

I did once basically explode a squirrel cage blower with the pressure side
of a vacuum cleaner. It ran so fast that the fins expanded and touched the
housing and became a bent up mess before flying out of the housing across
the room. I was not expecting that.