Or we are struggling to stop you refuelling your vehicles and rearming them in the area as we are burning through anti tank weapons to kill them, so we have taken out your fuel and ammunition supplies at source thus in effect cutting them off temporarily. It also means Russia will have to beef up their defence along their border to prevent it happening again, thus taking out more manpower and resources they can't use elsewhere and possibly will have to move their supply efforts even further back from the border and hence the front line..
I am surprised with the withdrawls around Kyiv and the possibility of the movement of equipment back through Belarus and into Russia to aid the south that they haven't dropped the odd rail bridge in Russia along the Belarus border.