In Black/White, when certain Pokemon are sent into battle some yellow sparks come out from underneath their feet. It seems like it's mostly large and heavy Pokemon and the sparks appear as they land.
I have seen it on Boldore/Gigalith, Scolipede, Golurk, Emboar and possible some of the dragons. I think it may only be trainer battles, not those in the wild.
Do they mean anything in particular?