The envelope generator section is an envelope generator used to dynamically shape volume and and/or cutoff frequency. If you're not familiar with the operation of envelope generators, here's an overview:
When MG-1 Plus sees a gate voltage from a note, the contour generator outputs a dynamically changing voltage, according to the settings of its stages. The Rise Time (i.e. attack) stage defines how long it takes for the output voltage to rise from 0 to 5 volts. Once the Rise Time reaches 5V, it moves to the Sustain phase if the Sustain switch is set to the In position and the holds at 5V until the key is released. If the Sustain switch is set to Out, it immediately moves to the Fall Time stage. The Fall Time (i.e. decay and release) slider defines the the length of time it takes for the voltage to fall back to 0V when the gate input voltage is removed, i.e. when the key is released.
Rise Time slider- Defines the length of time for voltage to rise from 0V to 5V when a key is played.
Sustain- When set to the Out position, following the Rise stage, the contour generator immediately moves to the Fall stage. When set to the In position, the envelope holds at 5V until as long as a key is held. When the key is released, it moves to the Fall stage.
Fall Time slider- Defines the length of time for voltage to fall from full sustain level to 0V when a key is released.
Tone Sources mode- This defines the behavior of the internal voltage-controlled amplifier, which acts as something of an automatic volume control and on/off switch for the final sound.
Contoured- Rise Time, Sustain, and Fall Time parameters are applied to amplitude.
Keyed- Rise Time, Sustain, and Fall Time parameters have no effect on amplitude - notes play in simple on/off fashion. This is useful when using using the Contour section for controlling filter envelope sweeps via the Filter Contoured Cutoff slider.
Continuous- Notes sound continuously. This is useful for drones and exceptionally lazy keyboardists.