Eric, spillover spas have to be fed by some water to overflow. Usually this involves shutting off the spa suction while keeping a return line open to the spa. Some spas have not only the jets, but another return (often looks just like a pool return) that can be used as a 'fill line' for when the spa equalizes to the pool level when the system is off (though the addition of a check valve in the system can counteract this).
One way or the other, there has to be more water entering the spa than is being removed by the pump for the spa to spillover, the ones you've seen that 'aparently' don't have the jets on either have a fill line or the flow to the jets is so small that you can't see the rippling or is insufficient to entrain air (they are going, but not bubbling).
HTH
Bookmarks