TL:DR
His hypothesis is that there are four separate forces fighting in Ukraine; Army with special forces, National Guard, Kadyrovites and Wagner. Due to the power play in Moscow, each of these might start pulling men away from Ukraine and back to Moscow, just in case any of the other three players makes a move on the throne. At some point this trend will have increased until there will be no fighting men left in Ukraine.
he's got a series of lectures on youtube about the history of Ukraine which is really interesting so far (only watched the first one) - still need to finish reading Bloodlands too...
Timothy Snyder being awesome, as always:
https://snyder.substack.com/p/how-does-the-russo-ukrainian-war?r=f9j4c
TL:DR
His hypothesis is that there are four separate forces fighting in Ukraine; Army with special forces, National Guard, Kadyrovites and Wagner. Due to the power play in Moscow, each of these might start pulling men away from Ukraine and back to Moscow, just in case any of the other three players makes a move on the throne. At some point this trend will have increased until there will be no fighting men left in Ukraine.