  The interlayer tunneling mechanism of the cuprate high temperature
superconductors involves a conversion of the confinement kinetic energy of the
electrons perpendicular to the CuO-planes ($c$-axis) in the normal state to the
pair binding energy in the superconducting state. This mechanism is discussed
and the arguments are presented from the point of view of general principles.
It is shown that recent measurements of the $c$-axis properties support the
idea that the electrons substantially lower their $c$-axis kinetic energy upon
entering the superconducting state, a change that is nearly impossible in any
conventional mechanism. The proper use of a $c$-axis conductivity sum rule is
shown to resolve puzzles involving the penetration depth and the optical
measurements.
