Helical fletched arrows create more drag at the rear of the arrow and also create more spin on the arrow.



So the effect of helical fletching is two fold.

One is that the extra drag allows the fletchings to have more control over the arrow than the broadhead does. You can only have 100% control of an arrow and it is far easier to steer something from the front than it is from the rear. So if you have three straight fletched vanes your vanes would have (hypotheticaly) 50% control over the arrow and the broadhead has the other 50%. Now go to a 2 degree offset and now your vanes have 60% control over the arrow and the broadhed only 40%. Now put on full helical vanes and now the vanes have 80% control of the arrow and the broadhead only 20%.

Two is that the added spin creates a gyroscopic effect on the arrow which promotes the stabilization of the arrow. Just like a rifle bullet. It is much harder for a spinning object to change course or direction whin it is spinning at a high rate of speed.