Dull burrs.. also if you dont have good control
of the grinder... high speed and sharp burrs will
will cut nicely but I dont like the high speed...
if it bites it will fly around the port multiple
times before you get it stopped.. I prefer a slower
speed and good control on the grinder.. on my electric
one the switch is on the end which is a push to turn
off... if I bump it off it will bite... but I tend to
keep my one hand right near the button/switch and the
other hand well forward.. this gives me better leverage
and control... I like the electric unit so I dont have
to listen to the compressor running.. and its a BIG
compressor
EDIT
Dull bits/burrs people tend to push harder and it
causes heat.. thats when things get weird.. if you
are leaning on them you tend to have less control..
sharp bits/burr you dont lean on them to get them to
do the work.. so with less pressure you tend to have
better control