Whether you soak the leather or not (as above), moisture has to work its way into the core, not just on the surface. If you have seen any videos or workshops with Jim Linnell, he doesn't soak the leather. He applies quite a bit to the surface the first time, using a sponge, then lets it sit and soak in for a while. He may apply some to the back if it is heavier leather and of course not glued to poster board, or taped. Then when the colour returns to near normal, he begins his cutting and tooling. If the leather is starting to dry out too much and doesn't produce a nice burnish while tooling, he will moisten the leather very lightly in the area he is tooling. It is faster than soaking the leather and waiting for it to dry enough to start working with.
Tom