Anyone knows how Adlib percussion works (compared to the normal sound channels)?

Souce info looked at: http://soundshock.se/phpBB2/viewtopic.php?t=21

The byte 0xBD of the adlib contains some bits concerning the percussion:
(src: http://www.shipbrook.net/jeff/sb.html#BD)
bit 5 - Set: Rhythm enabled (6 melodic voices)
Clear: Rhythm disabled (9 melodic voices)
bit 4 - Bass drum on/off
bit 3 - Snare drum on/off
bit 2 - Tom tom on/off
bit 1 - Cymbal on/off
bit 0 - Hi Hat on/off

I notice that there is a Rythm enabled with with sets channel 6-8 up for generating percussion.

So effectively the channel 6-8 are manipulated using bit 4-0. Can anyone tell me how this works? It seems the key_on of the channels override the rythm for that channel, forcing it into melodic mode?
(I'm trying to implement it into the Fake86 adlib code, which I use in my emulator)
Anyone knows how the random noise generator is used in this? (It seems the adlib has one to use with percussion?)