The right-hand crossover in the reference solution is only ever visited twice (if it gets visited at all): first when the B register underflows and second when the red ball passes through to get to the interceptor. That means we always want the first ball through it to go right and the second one to go left. So we can replace it with a bit. The benefit of using a bit is that we don’t have to use it at the point where the two paths meet. Instead, we can join the paths and put the bit further down to separate them again. That saves one part, because we don’t need the ramp on the right any more.