For educational and observational purposes, the footage below may be helpful, where Heifetz's mechanics on the violin can be followed in slow motion. For some, like myself, the exercise may prove frustrating, others may well benefit from it.
Each piece, or section thereof, is first played at normal speed with sound (except the first). The film is then slowed down by approx. 6-8 times, which makes its duration respectively longer. Excerpts chosen include bowing (up/down-bow staccato, spiccato), pizzicato (both hands), double stops and fast runs and shifts.
Of particular interest is Heifetz's habitual attack on the string, his use of the natural bow weight, the 'dig' into the string, the bow pressure fluctuations as it travels across the string (resulting in minute amplitude fluctuations, erroneously referred by some as "right hand vibrato"). Perhaps it is a kind of vibrato, as the amplitude does 'vibrate' inducing a fast oscillation in sound volume, but it is certainly not a pitch swing, as is the case with the left hand vibrato.