The Amp section controls overall amplitude (volume); this is mainly configured using the ADSR envelope generator, but controls for modulation amount and note velocity are also found here. .
Attack- Defines the length of time for voltage to rise from minimum to maximum voltage when a key is played.
Decay- Defines the length of time for voltage to fall from the attack stage peak to sustain stage setting.
Sustain- Sets the voltage level following attack and decay phases while a note is held.
Release- Defines the length of time for voltage to fall from sustain level to minimum voltage when a key is released.
Amp Drive- This sets the amount of drive into the amp section, and as you might imagine, it's used to overdrive the amp section. Hooray for distortion!
Envelope Generator Mode- This is the same as the Filter section Envelope Generator Mode switch - in the up position, the Amp envelope functions as a standard ADSR (attack/decay/sustain/release) envelope generator. In the down position, the Amp envelope is set to looping mode. This effectively converts the envelope generator to an LFO, with its controls affecting the modulation waveshape. The Attack and Decay controls are most effective here; be sure to set Sustain to a low value or the envelope won't loop at all.
Modulation- Sets the amount of mod depth from the Modulator section at top left. This is affected by the Mod Wheel switch in the Modulator section; for more info see the Modulator section.
Velocity- Sets the how much note amplitudes are affected by keyboard velocity. It works slightly "backwards" - increasing the the Velocity amount increases the velocity range, so note amplitude appears to decrease as the Velocity knob is increased (whack a key hard and it'll make sense).