I'm paraphrasing this from a social media post I read but have lost. The beauty of this move by Ukraine is that forces a dilemma upon Putin. If he lets the Ukrainian's run around taking territory and thumbing their noses at him, he loses face. If it tries to take it back there is a very real risk of damage / destruction of the gas pipeline that his economy desperately needs the western currency from - no pipeline, no USD.
The Ukrainian's know that he can't go in there all guns blazing (and certainly not tactical nukes) for risk of damaging the pipe. But he can't let them stay for the
PR disaster it is/is becoming. So with the added benefit of the
PR coup, they're reasonably safe from mass assault where they are and are drawing Russian troops away from where they could otherwise be creating trouble.