Bindings work in at least two axes. They are tested in those two according to some international standards. Boots are twisted until they release from the toe, and boots are pitched forward until they release. These tests are performed with various tools ranging from calibrated torque wrenches used manually to fully automated test benches.
Beyond the quantitative tests, the "torque test" typically includes a variety of other mandated functional tests and checks, and often any required adjustments short of remounting bindings.