Up to now every instrument has its own probability definition and plays on its own without listening to the other instruments. By defining groups, instruments can play a little more 'together'. Some instruments contribute their rhythm to a group and others listen to the group and play accordingly.
The contributors contribute their actual rhythm to the group, so the group maintains its own rhythm that is something like the sum of the rhythms of all contributing instruments.
The listening instruments modify their probabilities according to the rhythm of the group. If the listen value is greater 0, then the instrument will play at the same timing positions as the group rhythm. If the value is negative, the instrument will play at the timing positions where the contributors do not play.
The algorithm evaluates the instruments in the order they appear in the list. So contributors should be defined before listeners so its actual rhythm will be known to the listener. If a contributor is defined after a listener, the listener will listen to what the contributor played in the previous bar. An instrument may be listener and contributor at once.
Example 1: There are three different conga instruments: muted, high and low and you want them to play exclusively (only one instrument at a time). The first in the instrument list (eg the muted conga) will play its rhythm and contribute it to a group. The next instrument (e.g. high conga) will listen to the group with a value of -100, so it will play on those positions only where the muted conga does not play. It will also contribute its rhythm to the group. The third instrument (e.g. low conga) will also listen with a value of -100 so it will play only on positions, where none of the others play. See rrg2.rhy for an example of this.
Example 2: The open hi-hat shall go on some of the positions where the bass drum plays, it shall not play alone (without bass drum). In this situation the bass drum will contribute its rhythm to a group and the open hi-hat will listen to the group with a value of +100.