In Warmachine and Hordes, a pop and drop reduces someone's defense to a very small number by knocking them down or freezing them in place, allowing attacks to easily hit and destroy a model, particularly a warlock or warcaster. The "pop" is typically a feat due to killer feats released with warcasters in Warmachine: Prime, however certain other abilities can pop a models defense to an easily hit range.
Cygnar
Khador
Protectorate of Menoth