When the neck of a Champagne bottle is frozen and briefly opened during disgorgement, what actually pushes the yeast plug out?

Answer: The bottle's own pressure

The bottle's own pressure ✓ — Right: Champagne uses its stored pressure to eject the frozen plug of sediment. The neck is chilled so the lees leave as one tidy piece instead of scattering back into the wine. It is a neat reversal: the same pressure that makes the drink festive also becomes the cleanup tool.

Ice expands behind it — Freezing the neck makes a solid plug, but expanding ice is not the main ejector. The plug is pushed out when the pressurized bottle is opened. If pressure were absent, the frozen sediment would just sit in the neck and require a much more invasive removal step.

A pump pulls it out — A pump would be too clumsy for a bottle that is already a small pressure vessel. Disgorgement works because the sediment has first been collected and frozen into a plug. When the temporary cap is removed, the bottle's own CO2 pressure supplies the sudden push.