1984 Westone Thunder IA question

Hello all. Just taking out some guitars that haven't seen the light of day for some time and picked up my Red maple bodied Thunder I A. Guitar screams and is all original with the Hammer 1 pickups. My question is, is the guitar always in active mode and when I flick the switch its just a boost or is it only active when I flick the switch. I always thought its just when switch is activated but it just sounds so mean even with switch off I have to wonder. The control for the active switch is the tone type. Ill be keeping it out on the often played list with my ESPs and Arias for sure. Thanks guys. Anthony.