Inside the wheel ( behind the front face ) is a ring shaped thin plate ( like a spinner ) with the same set of voids.
When the plate is aligned, the voids are present - passing through the wheel face and the plate - but when the plate is rotated 72 degrees relative to the wheel then the voids close.
A set of small springs holds the plate open, but the plates are weighted off the axis slightly such that when the wheel speed exceeds a certain threshold the springs are overpowered and the plates slide into closed position. Slow down and they automatically open again.
Of course you could have any number of spokes, and if you make the spokes significantly wider than the voids, then the angle of rotation of the plate to close the voids will be smaller. By having the wheel spokes wider than the voids, the pocket for the plate can extend through a small portion of the spoke.
Another option for the mechanism would be a set of tiny electromagnets that exert a drag force on the plates, closing them. The electromagnets could be powered by the rotation of the wheels themselves. Carefully balancing the plates means a tiny amount of force will make them change positions.