This official Native Instruments video offers an introduction to Kontakt 4‘s “Authentic Expression Technology”.

In addition to the traditional way of sampling different volume levels of an instrument statically, Authentic Expression Technology is able to capture specific spectral characteristics of samples and gradually apply them to another samples in real-time, allowing you to seamlessly “morph” between different instruments’ characteristics. This allows for a new level of expression and realism when playing a sampled instrument.