To be honest, I'm not 100% sure what is happening chemically when leather is cased. I suspect it has something to do with saturating the collagen fibers that make up the leather. Perhaps it allows them to shift and compress around each other in a somewhat more fluid state. I've been curious enough to try to find some studies on the topic but I haven't yet found a satisfactory explanation.
In any case, my casing process involves dipping the leather in a casing tub containing a casing solution (mostly water but with an additive to assist with the casing) and then leaving them in open air for some hours. You can probably see that the water in my casing tub has turned kinda orange-y from the tannins left by the leather. It's important to get just the right saturation of water in the leather, not too much and not too little, so I've worked on a practiced "dip and shake" motion that's been working pretty well.
Once the leather's good and saturated it sits out on the workbench for some hours. This is to allow the water to penetrate deeper into the leather. We'll know this has happened when the leather has returned to the same color it was dry. Depending on the leather and the day this could take anywhere from 2-9 hours. I like to keep a dry cut-off from the side next to the casing leather as a sort of "color guide" so I know when it's done.
Once the leather reaches that point it goes into a casing bag. In times of yore this was a box made of wood and galvanized steel that could keep just the right amount of moisture inside the box. In modern times it tends to mean a big ziploc. By putting the leather in an air-tight container we're effectively able to "pause" the decasing of the leather and allow the water to soak through the entirely of the leather. In order to be sure the leather is cased through and through it's left in the casing bag overnight. I try to get all my tooling done on the following day but it's possible to leave the cased leather in the bag for several days if necessary. We wouldn't want to leave it too long though or there's a risk that mold could grow on the leather (or so I'm told, I've yet to have that happen).