Modern scopes have internal adjustments, with springs opposite the adjustment turrets. In theory, any shock to the system can potentially result in a slight movement of the internal lens cell, which is held in place by those springs. Any deviation, however slight, on the part of the springs to return the internals to the same point can manifest as a moving POI relative to POA.
Frozen scopes are where enterprising BR folks, in pursuit of the ultimate precision, take a fixed power target scope, and remove the turrets and springs and fix the internal cell in place. They then fabricate an external mount setup to facilitate holding / aiming the optic. The rear mount may have a set of precision micrometers, with locking screws opposite them. You unsecure the locking screws, make the adjustments, and re-secure the screws. The optic is held completely fixed or 'frozen' during the actual firing, with no opportunity for internal movement
The usage, in this situation, is as a reference optic where you *know* that the reticle cannot move due to anything other than how you position the gun. When you have both the reference optic, and the one under test, aligned on the same POA, you can start to see whether that Nightforce Competition 15-55x scope actually holds zero as well as you think it should. Does it settle in a few shots after an adjustment, or does it continue to 'walk' around for 6-8 shots. Does a Vortex Golden Eagle work better, or a Kahles K1050?
The short range (aka 'point blank) BR guys kinda started it ago, then the 1k BR guys refined the process a number of years ago, and then the F-class guys started getting on board. I'd say you probably need a gun and load capable of shooting down in the 0.2-0.3 moa realm (at distance) before you need to start worrying about the very small errors we're talking about here. The ELR crowd might be another group that could benefit.
