Switch effect? Switch effect from what device? The rectifying diode? That's the first part of the envelope detect. Switch effect from a negative feed back op-amp? Negative feedback is how you keep an op-amp linear. Switch effect from the BJT? A Bipolar Junction Transistor is a current amplifier. The base current gets multiplied by the Beta of the transistor to set the collector current until the collector to emitter voltage reaches saturation. (Not perfectly linearly, there is some compression before Vce saturates. But I'm not going into semiconductor theory 101 today!) What's left, maybe you think the resistors or capacitors will act as a switch.

Randall produced a very good, workable schematic. The only thing missing is the component values that a qualified or truly inquisitive person could figure out. If he doesn't have a second GA, I'll have to add mine to get him there.