Take a full deck in your left hand. Allow the cards to rest against your fingers just as they would if you were going to perform an ordinary overhand shuffle.
With your right hand, remove approximately a quarter of the cards from the back of the deck.
Bring your right hand forward and then rapidly brush the back of the smaller packet in your right hand against the front card of the larger packet in your left hand. Keep a tight grip on the cards in your right hand. Do this in such a way as to mimic the action of shuffling.
Move your hands from side to side slightly as you strike the cards together. The action of shuffling, combined with the slight movement, makes the deception very difficult for the spectator to follow.
From the spectator's perspective, you are performing a perfectly ordinary shuffle. Your hands are mimicking the shuffling action and the slight rocking motion of your hands makes it almost impossible for the spectator to see that the cards in your right hand aren't actually being released.
Replace the cards in your right hand at the back of the deck.