Cord Through Wrist
from the Science and Mechanics Magic Handbook

Effect: 
A length of light rope or cord about four feet long is tied around each of the magician’s wrists.  A smaller piece of rope or cord is tied firmly into a loop by a spectator.  The performer takes the loop, turns his back, and in a matter of seconds turns to face the audience again.  The small loop is now linked impossibly on the cord tied between his two wrists!  The cord around the wrists can now either be untied (a long process!), or simply cut to free the magician. 

Secret:
While your back is turned, pass the ring over one hand and up under the cord around your wrist.  Then, pass the outer side of the ring down over your hand again, outside the cord.  Viola!  The loop is now linked onto the cord running between your two hands. 

Alternate Method: 
A quicker way is to conceal a duplicate loop of rope up your sleeve before beginning.  In this case, you would want to tie both loops of rope to insure that they are identical in appearance. 

Using this method, when you turn your back you simply hide the loose loop in a shirt pocket and slide the duplicate loop down your sleeve.  Watch your angles when you do this – you may need to use a large cloth or towel to briefly remove your hands/arms from view.