The TIT team built a special refrigerator for the plane with a windows so they could watch the crystal formation in action. Then they placed large helium crystals in the bottom of a vacuum chamber and increased the pressure. When the crystals were zapped with acoustic waves, they crumbled into tiny pieces.
Normally it takes a long time to form crystals via Ostwald ripening - unless you have access to a superfluid. The crystal pieces were splashed with a helium-4 superfluid - a special state of matter with no viscosity, meaning it can flow without friction.
Per the paper's lead author, Ryuji Nomura, "Helium crystals can grow from a superfluid extremely fast because the helium atoms are carried by a swift superflow, so it cannot hinder the crystallization process." The low temperatures and high pressure in a microgravity environment didn't hurt, either.
In the end, only one 10-mm crystal survived - much larger than is usually achieved via this process, and featuring clear, well-ordered facets. As the scientists concluded in their abstract, "Gravity-free 4He crystals are now reality."