[This document was found in Bernard's notebooks. The notebooks date from during WW2 to the early 1950s. B.E.N.]
The key element in the drill is the action of the thumb. When a rope is grasped, the thumb is automatically put on the other side of the rope to the fingers. Consider a long rope with the ends out of sight to right and left. Let it be grasped by the right hand, palm up, and let the hand be twisted anti-clock until the knuckles are uppermost. Now let the loop fall below the hand. This has the practical effect of inserting the thumb in the loop, and unclutching the fingers would demonstrate the truth of the statement.
The thumb is now in the position of a bollard with one loop of a clove hitch on it. The rope going from underneath the loop must therefore be used to make another similar loop. This is the rope to the left of the performer. Therefore the hand is turned clockwise again preparatory to repeating the performance on the piece of rope to the left.
If the loop had not been dropped off the knuckles, and thereby effectively onto the thumb, this turn clockwise would undo the loop already formed. As it is the turn clockwise merely winds up both pieces of rope on the wrist, the loop round the thumb acting as purchase. This loop can only be destroyed by bringing the whole hand bodily in a clockwise loop towards the performer, and allowing the loop made by the right hand rope to slip back over the knuckles or off the thumb. If this error is avoided by keeping the body of the hand beyond the two ropes, ie the two ropes (twisted) lie between the performer's right wrist and his chest, then another loop may be picked up in the same way from the rope on the performer's left. This second loop when dropped onto the thumb or below the hand completes the clove hitch.
It is possible to carry on with "N" hitches by keeping picking up loops in the left hand rope.