The problem with your hoola hoop example is that it keeps moving after you go through it so that you have the same relative velocity. However, in the portal example, the exit portal is stationary, so in order to stay the same relative velocity to it when you exit is to speed up yourself, as in B.
The problem with your hoola hoop example is that it keeps moving after you go through it so that you have the same relative velocity. However, in the portal example, the exit portal is stationary, so in order to stay the same relative velocity to it when you exit is to speed up yourself, as in B.