Board Thread:General Discussion/@comment-7779702-20130404000656/@comment-1268838-20130404154612

It's called the "sword pressure" of the blade. Similar to when Ichigo first clashed with Ginjo after becoming a Shinigami again. Ginjo mistook it for Getsuga Tenshou, but Ichigo told him that it was nothing more than the shockwave being emitted by his sheer strength. We saw it even before that when Ichigo fought Aizen.

As for Engetsu, it would not be too hard to believe that it's a fire-type, or that the energy it emits at least resembles fire. While it has Getsuga Tenshou in common with Zangetsu, it is not necessarily identical in every aspect for both blades.