Your Irresistable Voice is a collection of vocal excercises designed to give you a more resonant voice with more tonal, pitch, and speed flexibility. It doesn't use too much NLP to teach but shows you how you can modify your own voice to deliver NLP language to somebody else. IPM lets you take on any behavior you like of anyone that you think is "magnetic". So without using your conscious mind to go through the YIV techniques over and over, you could use IPM to take on the behaviours of a speaker who's voice you'd like to emulate. Or you could get both, listen to YIV and then use IPM to let your other than conscious mind do all the work for you.
George
