In the above video, I have set the pattern to just 2 steps, one being filled with a sound, the other empty, just to make it easier to hear the effect. Initially, the envelope is just pure sustain, it's changed to something more refined with some attack and release. This envelope is then used to modulate the pitch of both oscillators (since oscillator 2 is off the mix, we are actually just acting on oscillator 1). As you can hear, the pitch of the sound follows the envelope: it goes up during the attack and goes down during the release.
Note that the envelope is always modulating the amplitude or volume: it's an internal or hard-wired modulation. Indeed, the volume goes up in the attack phase and down in the release. So you might have been modulating with the envelope all along without really knowing.