Timber!
Eren had dipped his hands into the sword's wraith's stomach through the wound he had created and executed his newly gained ability. Since the wraith didn't have any vitality, all he could extract was the wraith's mana.
Since he couldn't channel the dark blue mana through his mana core even though it had become compatible with his mana circuits, Eren couldn't use it the way he wanted.
The most he could do was release all the deposited dark blue mana in his mana circuits through his hands in one go– as if expelling it. That's what he did right afterwards.
Kachaaaaa!
The elf wraith's rib bones broke and produced a sound when the dark blue ice suddenly flooded his insides. The origin of that deposition was Eren's hands of course.