I think it increases the chance of getting a horse that's better rated with better trianing. It boosts all their traits by +2 (I dont really know what that means)
It is a guaranteed +2 boost across the board, but if the match sucks or random factor kicks you in the pants, you can still end up with a million dollar foal that never sees a LB. Personally, I would only use a sven on "special" matches, which to me is top ABLB.