The motion and placement of objects, sub-atomic or otherwise, are predictable, given their placement and motion and composition at any given point. Time travel involves reversing or accelerating activity that is predictable. This reversing or accelerating involves forcing the issue, as under normal circumstances objects proceed based on their surroundings, your familiar world. The forward press or falling backward is due to direction of flow, of what is just another element in your Universe. You are unaware of this element, as the steady forward progression is something you consider a given.
Nothing is by chance, and no motion is without reason or cause.
To you, in your perception, time is steady in its progression, but when the press of this ingredient is stopped, or diminished, what had occurred telescopes back in activity that is the reverse of what just occurred. The grip points we refer to are places in the orderly arrangement of particles where we can, in essence, set our sights and target, and stop during a time travel, else the unraveling of what has occurred would continue unabated. Grip points are predictable points where particles are structured in a particular pattern, and we aim for this pattern. This could be equated to so many ticks of the clock, or so many clangs of the hour bell, as the periodicy is predictable and reliable, like clockwork. All these matters are outside of the world you understand, but are as much a part of our science as working with solids and liquids and vapors are to you.