There's an echo with staff, spear and polearm like, 'lunges, pushing you off balance'. Supposedly this can lag the person being hit briefly. Is that just a rumor?

It's a small lag, but that being said, depending on what's going on a small lag may not actually be useful to you, since lag is usually not additive -- the biggest amount left essentially wins.

E.g. if I pincer someone (3 round lag on me) and you immediately after trip me (2 round lag on me), you can't even really tell that you lagged me because I'm already working off a bigger lag at that time.