The black ice uses...not silicon diodes, which are a .7v PN junction drop (or minimum forward bias, put another way). I "think" they're using germanium, which is much rarer, getting hard to find, and has a .3v drop. I think. If its not germanium, then its some other magic semi-conducting material, not selenium...which is about 1.5v drop.
Anyway, by using a low voltage drop diode, they can clip half a wave when it tops the drop point, lets say .3 volts. That voltage level is "just doable" in a hot pickup with ok playing. A really hot pickup might max at 1v, but you cant whack power chords all day. Reasonable playing might get you .3v or a bit more, which allows the clipping to take place. I'm not sure if they use one or two diodes (in reverse) but the whole sh'bang can be dialed in using a potentiometer like a tone control
