Hello Beautiful People, Fergal Here
This is very simple, and very useful.
When at a post office you can take money from their machine, even if you are with another bank. (I believe there is a charge but I don’t know how much)
First click “English Guide”
Then “Others”
Then “Cooperation”
Then put in your card and it works like a normal ATM.
This is useful, I have a post office near my house but no ATM. Knowing stuff like this is really useful every so often.
If you are in the Josei Centre, (Where you tend to go for your English clubs English Festivals and such) you are next to a Post Office that has these ATMs and knowing this is pretty handy if you need some money.
Well I hope this helps you out sometime
Good Luck
Ishikawa JET 
This is only a relatively new feature I believe, as it was only from April that people with postal accounts were able to transfer money to regular bank accounts.