Yamaha Electone EL-87 Owner's Manual page 103

Envelope Parameters
By using the modulators and setting them to various output levels,
you can create all kinds of sounds. These sounds will be static,
however, unless you have the output levels change in time. That is
precisely what an envelope does. It varies the output of an Operator
in time so that you can vary the tone or the level. If the output of the
modulator changes, the tone will change. If the output level of the
carrier changes, the volume will change.
Feedback
The output of a modulator can also be circled back to its input,
allowing it to modulate itself as well as the Operator below it. This
is called feedback, and it adds harmonic complexity to the tone.
Operators using feedback are indicated by a line that encircles the
right side of the Operator. Feedback cannot be directly controlled;
however, you can adjust the parameters of Operators that have
feedback to change the characteristics of the feedback sound.
99