1. When 5 or more Japanese people merge together to form one larger humanoid entity with all 4 limbs capable of independent movement. Much akin to a cheerleader pyramid, but the japanese never engage in this activity for entertainment purposes. It is solely used as an extreme last resort in order to attack or defend. Unlike a cheerleading pyramid, which takes practice, trust and great deal of familiarity with your team mates and/or system, japstacking can occur with 5 japanese total strangers. The reason for this is because japanese people in general are fit, trim and most of them are ninjas. This primal inner instinct or governing group mentality only occurs in the japanese, much like in army ants. When they merge together, it's not in the manner of smaller pieces of "The Blob" reconvening to form 1 big blob, but more in the manner of Voltron merging, but without the metallic machinery noises. They stack in a very calculated and mathematical manner, but without thinking or practice.
2. The human equivalent of a Transformer combiner or Gestalt (Devastator, Bruticus, Menasaur, Superion, etc.) As can be seen in the Drawn Together episode entitled "Gay Bash."
"Most of the Japanese ran when they saw Godzilla coming, but the students in Mr. Hashimoto's karate class started japstacking to defend the dojo from destruction."