x2. I think the 2BK, when timed properly resembles half of a fly kick and that's when the core muscles contract.
My core strength isn't good enough to do this for a long time. I notice there becomes a conflict between the core contraction and my desire/need to breathe. So I haven't quite figured that out yet.
I kick-toe-flick-down, and spear the opposite side and can feel the core muscles flexing across my body, but that's exactly when I should be inhaling to get an early bite of air. Mystery.