Warning: this is a subtle effect.
First test your sound output by pressing the appropriately named button ‘Test’. You must hear a clear ‘tink’ sound. If not, check your sound setup or go to another illusion :).
The phenomenon: Press buttons ‘1’ or ‘2’ ad lib and observe what happens: Try to determine the path of the blue balls. Are they crossing like so ‘X’, or do they bounce off each other ‘> <’?
Chances are: When there is no accompanying sound, the crossed path is seen, with the sound a bounce (‘> <’) is more likely to be perceived.
It was long known from the Gestaltists that two identical visual targets moving across each other can be perceived either to bounce off or to stream through each other (Metzger 1934). In 1997 Sekuler et al. demonstrated that a brief sound at the moment the targets coincide, biases perception toward bouncing.
Metzger W (1934) Beobachtungen über phänomenale Identität. Psychologische Forschung 19:1–60
Sekuler R, Sekuler AB, R Lau (1997) Sound alters visual motion perception. Nature 385:308
Shimojo S, Shams L (2001) Sensory modalities are not separate modalities: plasticity and interactions. Curr Op Neurobiol 11:505–509