Think of the laser wavetrain (red sinusoid) as being like a rotating disk with a slit near the perimeter. The disk is centered on the spindle of an electric motor, which rotates at relatively high speed, viz., "stroboscope". You can adjust its rotational speed by pressing your finger on the edge of the disk to slow it down.

If you looked at a florescent light (e.g., above your cube?) through the slit of the stroboscope while it was rotating, you'd see the blue and yellow frequencies associated with that emitted light. Normally, those frequency changes happen too fast for your eye to see, but the stobe appears to slow the down by shutting out most of the changes in between the time when the slit is in front of your eye. If you synchronize it just right, you can see either the bluish light or the yellowish light.

In the case of the electron, it is kicked out of the atom (Ar or He) by the UV laser and strobed with another IR laser. Instead of your eye being the detector (as with the florescent light), they use a special detector which measures velocity (momentum) distribution across it's surface. Of course, by the time the electron becomes free of the atom's binding potential, its momentum has started to spread out (like a diffracting wave--Heisenberg and all that) such that when they pile into the detector after being gated by each cycle of the IR laser stroboscope "slit', they interfere with one another and cause the wave pattern seen in their movie. In fact, the movie resembles the ripple pattern that would be caused by dropping a sequence of stones into a pond.