Originally thrown by Toad Ramsey, the grip was an odd one, even by today's standards. Ponder that, and you'll understand the rest of this explanation. A knuckleball is simply a ball thrown with little rotation, with one or more fingers curled so as to grip with the knuckles, fingertips, or sides of the fingers. For basic instruction on how to throw a knuckleball, not much needs to be said beyond this, except for the fact that most grips nowadays are with the fingertips. It can be gripped anywhere, any way, with success.
Funny things happen with a slowly-rotating ball.
A large pocket of swirling air develops behind it, and how the rotation is will change this pocket, larger, smaller, back and forth, and this pocket pulls against the ball, sometimes in one direction for a moment, then another. This pocket, or wake, is disturbed by air moving over seams or smooth surface. Air drags over smooth surfaces, but seams produce lttle swirls behind them, which act like ball bearings to allow the air to pass over easier and faster. Therefore, the orientation of the seams alternately trips and fills this wake behind the ball, giving it whatever action occurs. Another interesting thing is the fact that this wake causes a lot of drag.