The Earth’s solid inner core spins at a slightly different rate than the rest of the planet. Research from about 20 years ago showed that it rotates a tiny bit faster, moving ahead of the rest of the planet by about a degree per year, which meant that it would lap the outer part of the planet’s rotation every 360 years or so. This is referred to as superrotation. New research shows that the core can speed up and slow down how fast it spins.
However, the core didn’t stop and then start spinning in the other direction. Instead, relative to the mantle and crust it changed direction, going from moving slowly ahead of us to moving slowly behind.
This is because the inner core is solid, made mostly of iron and nickel. The iron forms huge hexagonal crystals that are not smoothly distributed across the core and a division between the western and eastern hemispheres. Thus, the gravity from the inner core isn’t smooth either, and the different hemispheres tug on the Earth’s mantle in different ways. This can shift the way the core rotates, slowing it down and speeding it up over time. This is rather complicated and not totally understood. The inner core is a ball of iron hotter than the Sun and under huge pressures that’s 2,500 km across and buried under a shell of molten iron 2,200 km thick that itself is under more than 5,000 km of hot mantle rock.
https://www.syfy.com/syfy-wire/bad-astronomy-earth-inner-core-spin-constantly-changing