Compression fields were a type of technology used to fit large objects into small spaces though miniaturisation. It was similar in use and capability with moleculon nanoscaler technology.
Miniscopes used compression fields to keep specimens inside them miniaturised. Specimens extracted from the field resumed their original size. (TV: Carnival of Monsters)
The Teselecta used a compression field to miniaturise its crew to the size of an insect. (TV: Let's Kill Hitler)
Members of the Slitheen family of Raxacoricofallapatorius used compression fields in their collars to allow them to wear human skins and disguise themselves. However, this had the side effect of frequent gas expulsion. Initially, it was limited to humanoids of large builds. (TV: Aliens of London/World War Three) Compression field technology soon advanced to the point where they could fit inside the skins of thinner humans and life forms the size of footballs. Also the gas exchange issue was fixed. (TV: The Lost Boy, PROSE: The Monsters Inside)
Leef Apple Glyn and Tree Lorn Acre of the Slitheen-Blathereen family wore collars. While posing as law-abiding Blathereen, they claimed that they did not wear skin. (TV: The Gift)
A pair of Raxacoricofallapatorians were seen wearing collars at the Zaggit Zagoo bar. (TV: The End of Time)
One of the Slitheen's compression fields was acquired by Henry van Statten and kept in the Vault by 2012. (TV: Dalek)