The paper is really nice and readable. TL;DR: Freezing the phone makes the RAM static and not clear on reboot, giving you time to sideload their custom recovery image that iterates the ram and looks for AES encryption key patterns.
Actual freezing. Like, putting the phone in a freezer to create sub-zero temperatures.
Under many circumstances, the attack worked at room temperature; one main reason for using low temperatures was if you needed to physically move the RAM chips from one machine to another.
from https://www1.informatik.uni-erlangen.de/filepool/projects/fr...
> The remanence effect says that RAM contents fade away gradually over time, as slower as colder the RAM chips are.