Dave Smith Instruments SYNTHESIZERS

Could someone explain the Drone phenomenon on the Evo ?
I take any patch, turn up the filter or vca
and let the tweaking begin !  It really opened some new 
territories for me.  I'm still a student of the electronic world and  i 
was wondering why is the Evolver so good at this and not other 
synths ?
And how / why does it work ?

Thanks for any comments
Volt