Okay so essentially how this works is...
Let's say you have 1 stack withdraw per day.
This example we're going to use Greater Celestial Essence.
You have 1 Greater Celestial in your bag, there is a stack of 10 in the guild bank.
Simply drag your 1 Greater Celestial onto the stack in your guild bank and it swaps them out without using your withdraws.
You now have 10 Greater Celestial's and the guild bank has one. It works with almost anything that is a complete stack. (Chaos orbs/cloth/etc.)
Cheers,
Fredalbob