The abrasive surface marks the special rotation of the joint in the middle of the mouthpiece which has been turned forward by 45 degrees. The special surface is not made of a different material but a roughened surface of your Sensogan, Aurigan or stainless steel bit which helps to distinguish the angled Turnado bit from a standard single jointed bit. The advantage of the special angled Turnado bit is that it allows an even distribution of pressure on both sides of the tongue. Standard single jointed bits have a production related problem: one part of the mouthpiece is longer than the other which results in stronger influence on one side of the tongue. With long-term use of the bit, the roughened surface might decrease or disappear which has no influence on the effect of the bit. Click here to get to the extensive range of Turnado bits.