  High velocity jets are among the most prominent features of a wide class of
planetary nebulae, but their origins are not understood. Several different
types of physical model have been suggested to power the jets, but there is no
consensus or preferred scenario. We compare current theoretical ideas on jet
formation with observations, using the best studied pre-planetary nebulae in
millimeter CO, where the dynamical properties are best defined. In addition to
the mass, velocity, momentum, and energy of the jets, the mass and energetics
of the equatorial mass-loss that typically accompanies jet formation prove to
be important diagnostics. Our integrated approach provides estimates for some
key physical quantities - such as the binding energy of the envelope when the
jets are launched - and allows testing of model features using correlations
between parameters. Even with a relatively small sample of well-observed
objects, we find that some specific scenarios for powering jets can be ruled
out or rendered implausible, and others are promising at a quantitative level.
