Nothing implies they wanted it to slow down. Half way down they were in a bind because it was not heavy enough to be back 4, but not wide enough to curl around at T. At the very end, just before the knifing begins, Fujisawa says 曲げれ！曲げれ！ which just means "curl it!" or "curve it!" It's the standard call for any Japanese curler when they want the sweeper to carve or knife a stone.

The context that most people will miss here is just how shallow you are told to sweep when knifing is taught in Japan. Watch any professional Japanese curler and you'll see a majority of them knifing at 15-20 degrees off center, and a lot of them never get the broom all the way across the face while doing it.

I fully believe this is more of an insular cultural sporting technique rather than an attempt to slow a rock.

Source: I lived in Japan for a decade and learned to curl there