I can’t explain in great detail what is happening here, but it seems that the people performing the experiment are dropping the pressure inside the glass, causing the boiling and freezing points to come closer together. This phenomenon is called the “Triple Point.”

In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.

[Tom Enstone | Via Neatorama]