http://www.playprosoft.com/tips/tb_3.html

The above link should lead to a series of photos demonstrating the "correct" plec hold method BUT - as stated previously, there are problems with these ilustrations. First, it is impossible to see the finger joint that the plec is resting against, and secondly, I believe that the middle finger joint should not be pointing downwards. If this does not happen, the hand can become tense.