It has four "banks" of space for data. Each one can hold as much as a Duo/CD-Rom/TurboBooster Plus/etc.
You put the card in your system and it turns on like any other hucard.
You have 3 options after you press "run."
1)You can put
all of the memory on your Duo into one of the four banks. In this case the original files also remain on your Duo, so they now exist in two places.
2) You can put all of the data in one of the four banks into your Duo. This will erase anything you currently have saved on your Duo.
3) You can switch all of the data on your Duo with all of the data in one of the four banks. With this option you don't have to worry about erasing any of your saves.
So...with a Tennokoe Bank you can fill up all of the space on your Duo and all of the space on the four "banks" inside the card. This gives you the equivalent space of five Duo's to screw around with. This is crucial for those SCD RPGs like MM3, DE2 and Dragon Slayer.
The main problem is there is no way to view the files in the card before you switch. So you either have to remember which of the four banks your Ys files are in of figure it out through trial and error. There's also no way to edit out individual files while they're on the card without transferring them to your system first. There's also no way to merge data from different banks into one. This is a pain in the butt because I have my high-scores from dozens of different shooters and action games scattered throughout my two systems and two Tennokoe Banks.
If you haven't seen it yet, Darkman long ago translated the Tennokoe Bank comic.
It can be viewed here:
http://www.darkcityproductions.com/duo/Tenokoe.shtml