What's the deal with the Envoice tube?

[Markd102]

I borrow a mate's Envoice every now and then but I've never had the chance to read the book.
My question is...... Does the tube only come into play at the compression stage? The saturation control is in the comp section, and that "lovely little yellow light" only goes red with the comp enabled. Is my assumption correct?

Thanks
Mark