You really can't root prune while it is in the container, how would you get to the roots 1' down?

As for timing, I am not sure about your tree, but in most cases you will want to do it in the spring when it resumes active growth, but some can be done in the fall as well. Mainly you just want to avoid doing it in the heat of summer. When root pruning try and spare as many of the thin roots as possible, those are the feeders. The thick, large roots can be pruned more as they don't do much for the tree other than anchor it in the pot.

Root pruning an in-ground plant might occasionally be undertaken as preparatory measure prior to lifting a plant or to reduce vigor temporarily. This type of root-pruning is done spatially, usually in a circular pattern around the stem (trunk). Root pruning a containerized plant should be done selectively rather than spatially. As roots get larger, they become woody & less effective at absorbing water and nutrients from soil. In in-ground plants, these roots serve to anchor the plant and keep it from toppling. In containerized plants, they are less than useless. Not only are they ineffective at absorption, they take valuable space that would be better used by more efficient, fine rootage.

You cannot effectively root-prune a containerized plant without removing it from the container. If you were to attempt some sort of spatial pruning, you would remove fine rootage along with woody roots, leaving lots of succulent tissue in the container to rot. In addition, using a sharp tool is particularly important in pruning roots of container plants. The exercise of root-pruning without removing the plant would cause excessive trauma to roots - think of the force required, even if the tool is sharp.

Apple trees are very vigorous & tolerate bare-rooting and root pruning well. Best results will be realized if you first unpot and saw the bottom half of the root-mass off just before buds move in spring. Then, remove all the soil from the remaining roots and selectively prune 1/3 of the largest roots remaining. Repot in a fast draining soil, using chopsticks or a similar tool to work soil into all air pockets in roots. No pruning of top is necessary, but it is an excellent time to undertake any radical pruning of the canopy.

If you only prune off the outer edge of soil & pot up, the old soil in the root mass will eventually become extremely hard. Roots enlarge and grow into each other & the hard soil makes expansion of the corky cambium impossible. The effect of both these conditions is the same as girdling roots. The first symptom of the need to prune roots is diminished branch extension. I can look at a potted tree & tell how old it is and what years it was repotted by looking at the distance between leaf bundle scars. As the tree becomes root-bound, the scars get very close as extension is diminished. After repots, the scars are farther apart as vitality is improved after a repot. The second symptom of the need to repot is noticeable decline in overall vitality and death of individual branches as the root(s) primarily responsible for feeding the individual branch(es) is/are strangled. Eventually, the tree will decline to the point where energy levels are diminished so much the tree cannot keep its systems orderly, resist infections from pathogens or insect attack.

Al