What is Ki Reservoir?
  -The Ki Reservoir is a Ki Storage potion, allowing you to store up to 1 billion Ki, equivalent to 1 Ki Reservoir.
    This potion can be stacked up to a maximum of 255/255. The Ki Reservoir works only with stats that
    have reached 9999.

You can obtain the Ki Reservoir by clicking the "+" sign on the right side of your "Gain Ki" , or by enabling it in Auto Play. Don't forget to click "Save" afterward.

To use it, simply click on the stat you want to add it to, and it will deduct the stored Ki from the Ki Reservoir.