There are several secrets to successful slurs. For hammer-on slurs, the first rule is to prepare the fingers. Each finger must be in place just above the string to play. Many contend that a significant distance is required to produce a clearly audible sound. However, I would invite them to consult the documents available explaining the famous one-inch punch of Jeet Kune Do master Bruce Lee. He showed that he could throw back an opponent with a punch initiated from only one inch away. Using this technique, he demonstrated the well-known principle in physics (kinetic energy) that it is not necessarily distance that affects force, but the speed at with the object is moving.
So the secret is a good positioning of the hand and a quick descent of the finger led by the first joint. The positioning of the finger must be optimal thanks to the use of the second and third phalanx so that the attack is perfect and there is no energy loss.
In the case of pulled slurs, the fingers should be placed on the string, as the finger that will play the second note is already in place. In this case, it is the second phalanx that act as an engine.
Again, speed is the secret. The finger that makes the downward movement can also use the bounce of the bottom string (which then serves as a springboard) to change direction.
In achieving the slurs, we can also use the pronator and supinator muscles to give a boost to the hand. Under no circumstances should the hand pull the pronator and supinator muscles. These are the muscles that power the hand.
© Jean-François Desrosby (D.Mus.)