One thing to remember when filling shocks, some shocks are preloaded with nitrogen gas, if the shock still has pressure you have to be careful when drilling the hole or you probably will be soaked with oil
The loss of the pressure in the shock does NOT affect it's damping capability in normal tempratures, it only affects the foaming in the shock in high tempratures.
But the pressure also gives a little "help" to the spring and when you remove this pressure, the car has teorethically somewhat "weaker" spring, but this is compensated by the far better damping capability in the rebuilded shock.
As I know every shock has a similar system with inner/outer tube and a piston with holes/valves, so this method could be used on the majority of shocks (as a "economy" method
).