Hippie's Dunk tek is a simple yet effective way to extend the useful lifespan of cubensis cultures such as PF cakes.
This tek should be used whenever the cakes/casings dry out. Also it can be given after full colonization to replace water lost so far.
The tek itself is quite simple. It essentially consists of submersion of the cake/casing underwater for 12 to 24 hours. Don't worry, that will not harm your cakes/casings. This is best done under refrigeration, if possible.
One can even dunk cakes/casing that already have small pins safely. Just don't overdo it, by 48 hours underwater they will be dead.
As for what kind of water to use, natural spring water is best but you can even use it straight from your faucet if need be.
Temperature during the dunking should be cold, or as cool as possible and still above freezing.
Time spent dunking should be not less than 6 hours for minimal benefits. 12 hours is about right for dunking in between flushes and at birth but 24 hours is the maximum for full re-hydration of nearly spent cakes.
There are at least 2 options for how to dunk. My preferred method is to dunk each cake individually by placing it in a jar, filling with water, then screw on the lid to keep the cake submerged. This way is good for small batches, and has the advantage of keeping each cake isolated so no contaminants are spread from cake to cake.
For larger batches, you can simply place several cakes in a large pan or bucket, cover with water and weigh down the cakes by placing a weighted down lid, plate, etc. on top of the cakes.

After dunking, drain well, gently pat dry with paper towels, etc. and return it to the fruiting environment.