You replace the jack cup with this unit and keep the 9v battery under the pickguard.
They have been making them for decades. Used to be rectangle shaped and called stratoblaster.
I really like it, sounds great and well made.
I don’t run it for overdrive or distortion just to give a boost.
Someone described it as sounding like you installed a new set of strings, I’d somewhat agree with that.
Famous players include Lowell George, Nils Lofgren, and maybe Jerry Garcia.
Here is a link to a page that describes it pretty good: