No, I thought it worked like that once too, until manao chastized me.
All macroblocks with quant > 15 are always processed.
Like, say if normally, a block with twice as many (high frequency) details was smoothed twice as hard as one with fewer details. By lowering beta you could make that be smoothed 1/4 as hard, or by raising it make it smoothed just as hard. So it changes the way the smoothing scales, which is already based on the detail present.
This is what I get out of it, so far, anyway.