The alternate function in the NN-XT sets up keyzones where samples are not repeated. Rather they are played in rotation. Say for instance you have a multi-sampled snare drum. Set the velocity ranges in an overlapping fashion.

Now as you play snare rolls, paradiddles and such the samples will not sound like a machine gun. It is important that the 2 lowest velocity and 2 highest velocity samples range to 0 and 127 respectively.