(Jon of the Clue Crew shows himself on the monitor, swiveling around in a chair with dumbbells in his hands, extending and contracting his arms.)  When I draw my arms inward, my angular velocity increases because my angular mass decreases; the product of those two quantities is called angular this
In many cases they're able to increase their speed without 
pushing off of anything simply by tucking their arms in to add any extra force to 
the spin.