Starting and stopping
A vibrato you can only do "on" is not under control yet. The next skill is starting and stopping it on purpose — playing a note dead straight, then letting the shimmer bloom, then taking it away again, all while the bow keeps moving.
Straight, then bloom, then straight
How to play it
Slow, even bow throughout — the bow does not change when the vibrato starts or stops. Only the left hand switches. If the bow lurches when the vibrato kicks in, that's the thing to fix (see 'keeping the centre').
Why this matters — the "late" vibrato
Starting a note straight and letting vibrato bloom on it a moment later is a real expressive device, not just an exercise. It's how a Baroque or Classical player often approached a long note — the note speaks cleanly first, then warms. It's the opposite of the modern habit where the vibrato is running before the note even sounds. Being able to do it means being able to choose when the shimmer arrives — which is the whole point of Movements E and F.
Try it
Try
On one long note: four slow bows. Bow 1, all straight. Bow 2, all vibrato. Bow 3, straight then blooming to vibrato. Bow 4, vibrato then settling to straight. If bows 3 and 4 are hard — if the vibrato can only be all-on or all-off — spend a week just there. That switch is the control.
Hear it
Listen to almost any phrase in a period-instrument recording of Bach (Rachel Podger, say): notes often speak straight and then a little vibrato arrives on the longer ones. Then a modern Romantic concerto for contrast — the vibrato is simply always there. You're learning to do either on purpose.
The usual mistake
A vibrato that starts the instant the bow moves and never stops — you've got a switch stuck in one position. Or the reverse: a vibrato that dies the moment you think about anything else, especially the bow. Both mean the same fix: practise the deliberate start and the deliberate stop, slowly, until "straight" and "shimmering" are two settings you move between at will.