The Twin’s effects loop has the usual Preamp Out and Power Amp In jacks, but it also has the Power Amp Thru jack. This is a tap taken just after the loop that can be used as a line-level output, but that is specifically designed for slaving other Twins in series. A regular guitar cable is used from the Power Amp Thru of the master amp to the Power Amp In of the slave, and from there the slave’s Power Amp Thru is connected to the next slave’s Power Amp In.
Another novelty here is the three-position level switch for the loop, with -16dbV and -7dbV for “guitar” level and +4dbV for “line” level. The idea here is to match the level of whatever you have plugged-in to the loop for the best signal quality. Most rack unit effects will run at +4dbV.
When using the loop to slave other Twins it is important that all amps have the exact same loop level setting.