Martelé bowing is a hammered stroke with a crisp bite at the beginning of the note, followed by a complete release of the fingers which become totally relaxed as you follow through.
A good way to start learning martelé is by holding the bow only with the thumb and index finger, and pushing the stick into the bow hair, with the index pressing down while the thumb presses towards the ceiling. Once this feeling is well understood, you can start adding the other fingers.
The player must be careful not to crush the sound at the end of the note : this is done by totally releasing the fingers on the bow as soon as you start moving.
Martelé is easier to learn near the balance point of the bow.