My understanding is it's a simple padded input (6 db, perhaps?), though I'm no tech so perhaps others will chime in with more specific info. Many types of gear have available padded inputs, e.g., most PAs (for regular and condensor mics), acoustic guitar amps (passive and active PUPs), and of course, electric guitar amps.If you Google "twin reverb second input" and/or "amplifier padded input," I'll bet you'll find lots of discussions. What I'd do in your position is take 5 minutes to experiment with yours and listen. This will tell you more than anyone else's descriptions.
Disclaimers: I'm happy to share whatever relevant gear knowledge or experience I have. Genuflection/payment/proof of worthiness neither sought nor accepted. I don't design, build, market, sell or endorse for pay any new music product or service. I occasionally sell personal used gear. I am an unpaid endorser of SoundScaper Productions and Godfrey Daniels Guitarworks.