If I understand correctly, instead of the usual kind of boxing, they use a separate type tag (one byte) and support union types with up to 255 other "special" enum values. The type tags and data are stored separately, so instead of an array of boxed values, they internally have two arrays, one for type tags and the other for data.
Apparently this can work well with SIMD instructions.
That's pretty cool. There's a lot that a couple of us on the Pony core team like about Julia from our limited knowledge. I'll have to look into that more.