Technical Question

Hi. I think this is a superbly-made short, even if you don't find the subject matter of value or find it objectionable. But that's besides the point. I'm just curious about how they made one scene:

Apparently none of the special effects were done in post-production, it was all done in camera. So I'm curious, for the scene where time has stopped and everything and everyone appears to be frozen, how did they manage to have the milk frozen mid-air when someone else is moving within the same frame?

If anyone knows I'd love to know how it was done. Thanks.