It makes perfect sense if you look at the maps.
Russians had previously maintained positions within artillery range of Kharkiv, and extensively bombarded Kharkiv from these. They have also maintained occupation of a buffer zone on the west bank of the Donets east of Kharkiv.
The Russians have either deliberately withrawn from these areas in the last week, doing a fighting retreat, or have been defeated and forced to withraw, as per the Ukrainians. Believe whom you like, the upshot is the Russians are no longer there and the Ukrainians are. This withrawal is an unambiguous benefit to the Ukrainians.
This area is where the first of the new NATO standard artillery (American M777 howitzers) started firing, right around the time that this counteroffensive began.
Did the Ukrainians just throw the new stuff to reinforce what was already their main effort, or did the new Artillery capability blow open the way for them to advance?
Time will tell if the game has changed.