This also occurs with any prop when it is turning slower than it would to maintain speed (thrust). So, it acts as a brake.. adds drag and slows the plane down. Pattern pilots like this function because they can control speed better when using props selected for more drag during deceleration.

Generally speaking, the drag will increase with length but decrease with pitch (the more pitch, the less drag). There was a study done on the subject, and if you are interested, take a look here... http://www.peter2000.co.uk/aviation/misc/prop.pdf

It's a tad bit complicated, but informative.



CGr.