It is my contention the swiping works because it is to fast to be recognized as an actual rhythmic activity. I have seen that if you do it too slowly it becomes know-able that is to say felt as something separate from other actions. As has been explained the direction of the swipe must be done on outside string changes so as not to fight the string. The sweep of the pick touches the string so quickly that the tree does fall in the forest silently.
However descending inside string changes(probably ascending as well))are possible also to swipe, even at slower tempos. This means that on inside string changes it is easier to do two downstrokes “backwards” as it where because the feeling of the rhythm is accomplished, whereas so called “inside string picking” feels empty(that is to say the upstroke does not appear to exist). This is because the strings resistance to the pick moving “through” it is basically infinite but still providing enough momentum for a downstoke on the same string for fours, sixes, scales etc to ascend and come back until the same situation occurs again.
I understand that rotation is supposed to solve this but as has been pointed out the significance of being able to rotate cannot be worked on slowly because it’s not relevant(Catch-22 anyone?).
The solution is it hand but this issue probably vexes many many players who cover it up in complexity and the belief that more time will make the difference. This is probably not correct.