When an assist is observed, where does the energy come from to boost the BB higher up the inclined glass surface? The Jupiter magnet connects its revolving motion to the BB, as the BB rolls by. The BB picks up motion from Jupiter. If the BB rolls by a little faster than the Jupiter magnet's "escape velocity," then the boost is successful. If less, the BB "crashes" into Jupiter and sticks to the magnet. Of course the motor supplies the revolving energy for the Jupiter magnet, while the real Jupiter draws upon its huge primordial momentum as it coasts along in orbit.
Finer points: At the top of this page is the question, "...what's the difference?" Only a subtle difference. The simulator's Jupiter and spacecraft are of similar mass, and the magnet does all the pulling. As it does, the Jupiter magnet feels a backward tug as it supplies the energy to accelerate the BB. In interplanetary space, the scale is so much larger that gravitation makes the connection, Jupiter pulling on spacecraft and spacecraft pulling on Jupiter. Another fine point is that if you were to place an accelerometer on the BB, it would register acceleration as the boost takes place on the simulator. This would not be true on a spacecraft. A spacecraft feels nothing but continuous freefall as it picks up a large increment of speed during a gravity assist. This is because there is basically no gravity gradient across the spacecraft or its accelerometer.