The solid inner core sometimes spins a little faster than the rest of the Earth, and sometimes spins a little slower (the changes are very gradual, and are related to the Earth's magnetic field). For many recent decades it is believed to have been spinning a little faster.
All that has happened is that the inner core has slowed somewhat, and instead of spinning a little faster than the rest of the Earth, is now apparently spinning a little slower. That's all. It might affect the Earth's magnetic field a little bit.
IT HAS NOT "REVERSED COURSE". What a load of poo.
It’s reference points. Their axis of rotation is not the same, and more importantly, they are spheroids, not cylinders, so a significant wobble can occur at times as the flow becomes unstable.