Board Thread:Questions and Answers/@comment-139.78.10.104-20160828063446/@comment-27384850-20160828070641

That's not a Sword Skill of any kind, it's literally her just blocking Kirito's and Asuna's attacks. The dust was just there for effect, Kirito says in the book that he said something about her being a resident of VR and she responded with a smile and a super-fast charge, the dust cloud was likely added here as to hide Yuuki herself as we had not yet been introduced to her. There's also no dust in the fight against Asuna, at least no big dust cloud like in the anime.

TL;DR: It's just a normal parry, anything special about it comes from the anime wanting it to look cool.