Again, I am using only parts in the box. Add a cog to a gear bit then stick the black rod of an interceptor through the hole of the cog.
The black rod of the interceptor reflects ball s from either side back the way they came.
The cog allows the obstruction to be moved!
A gear bit (weighted using another cog or ball) controls the position of the black rod. When down, it is a reflector. When up, balls from the left pass through like a normal crossover, balls from the right are trapped until released by another ball hitting the gear bit. Photo below…