The pull
Simulator
This is the only control that matters. Drag it to keep the mark on the subject, and watch the marks rather than the picture. By the time the monitor looks soft you are already late.
Marks, and how much room they give you
Pulling focus is not a reaction, it is a plan. The depth of field tells you how much margin you have; the marks on the floor tell you where to be; and the monitor tells you where focus was a moment ago.
Marks come first
A focus puller watches the actor and the marks, not the picture. By the time softness is visible on a monitor the mistake has already happened, and the correction will arrive late and then overshoot.
The depth of field is your margin
Everything about how hard a pull is comes down to how many centimetres of room the stop and the lens give you. Two stops down can turn an impossible pull into an easy one, and it is always worth asking for before the lighting is finished.
Long lenses are the hard ones
At the same framing a long lens has about the same depth, but the subject crosses it faster in screen terms and any error is magnified. A wide lens forgives; an 85 at full aperture does not.
Lead the move
Focus has to arrive with the subject, not follow them. That means starting the pull before they do and easing into the mark, which is why rehearsal matters more than reflexes.
Speed changes everything
A walk gives you time to be smooth. A run at the same stop gives you the same centimetres to work in and a fraction of the time to cross them, which is the moment to ask for another stop or a wider lens.
It is a department, not a knob
Marks laid by the second assistant, a rehearsal with the actor, a distance read off a tape rather than guessed, and someone whose only job is the ring. Everything above is how that job is made possible.