What is this? What is it called, and what does it do?

[KarenColumbo]

Other people read the Classics to wind down, I read schematics of old amps - and from time to time my weary eyes stumble upon extraterrestrial things like this (marked red for your convenience):

I like the look of it. But what is it? What does it do? How would I name it?

[PRR]

The JFETs are switching the cathode caps.

[KarenColumbo]

The JFETs are switching the cathode caps.

Thanks! Not as mysterious as I hoped, but another bit of learning done.

[bool]

Switching these caps of course being for more gain.