Seems there is a change in the Earth's rotation rate on the surface when Glaciers melts and the weight of the water goes towards the equator, and like a spinning skater, when you have your arms outstretched and bring them inwards your body spin rate speeds up.
Same thing happens to Earth. The water at the poles in the form of ice is rotating relatively slowly compared to the equator. So when the water melts and goes to towards the equator, the earth's rotation rate speeds up.
But not the core. Therein lies the difficulty.
This guy is studying the results of the spinning Earth and the changes in the spin rate and the change in the spin rate of the core.
Obviously, if the core and the surface rotate at the same rate, things are smooth.
If they are different, then mantle movement can take place, causing among other things, earthquakes, volcanism and so forth and he is saying, also sea level rise.