"A slow to moderate speed bouncing stroke."
In spiccato, the down-bow and up-bow actions should be the same:
Use the same bow speed.
Start and finish at the same distance above the string.
Follow the same flight path in the air.
To encourage the feeling of letting it happen in spiccato, begin working with expansive movements - aim for the double stop of open D and A-strings and use a large, loud stroke so that you can concentrate on the movement and not be too concerned about precise landings.
The give in the bow hand (especially the 4th finger) is very important, as the bow needs to be supported with ease while in the air (see Bouncing Bowings - Ricochet - Add Bowstroke').
Practising without the 1st finger on the bow can also be useful as a tight 1st finger makes it difficult to play an even stroke or make a resonant sound.
Holding the bow at the balance point in the early stages can help deal with the weight of the bow in the air.
Although it is an off-string stroke, the first note of a spiccato passage is often best started on the string.
As with many other bowstrokes the down-bow in spiccato can be naturally stronger and heavier than the up-bow, leading to unevenness. Practise spiccato starting on the wrong bow, for instance Kreutzer's "Study No.8" starting on the up-bow, to help improve evenness.
In spiccato the main action, the initiator of the stroke, can be the forearm, the upper arm or the hand.
The forearm is the most useful of these three as the stroke is at its straightest, it relates most clearly to the détaché stroke and it can function at a variety of speeds, bow lengths and hair inclinations.
When learning spiccato it can be difficult at first to do spiccato with a forearm action. The best sound for slow spiccato is made playing in the lower third of the bow, and when we play in the lower third we will instinctively use the upper arm rather than the forearm.
One solution is to develop the spiccato in the middle of the bow where the forearm works with ease (even though the sound will be somewhat brittle and the bow will bounce quite high).
When the stroke is established then it can be moved lower in the bow to allow for a lower bounce and a longer bow.
The upper-arm spiccato is a more crooked stroke than the forearm, but has the advantage of working well in the lower third of the bow. It can however work quite effectively within a limited range of tempos and tone colours.
The hand spiccato is the least useful action, as it involves quite a lot of change in hair inclination, and it is difficult to change the length or dynamic of the stroke.
Bouncing Bowings - Ricochet - Add Bowstroke
Definitions reprinted with permission from American String Teachers Association.