Still on the first page:
http://www.dakkadakka.com/dakkaforum/posts/list/598738.page (But to answer your question, only if the enemy has a higher or the same initiative as the
GUO and they are able to reach base contact with their Pile In move, if they have a lower Initiative the
GUO will not be in base contact with anything so he can not swing on his initiative).