The consumption of the shed vortex from the preceeding blades (it is called Blade Vortex Interaction) is what you feel when decelerating through translational lift.
Some helos also get the shudder when accelerating thru ETL, especially when the acceleration is made level, and not from a climb. Same cause of course.
From the first source below, "Under certain conditions of powered descent or vigorous manoeuvring, rotorcraft blades pass through the wake and trailed tip vortices from previous blades. This interaction of the rotor blade with the tip vortex of a preceding blade has been identified as a significant source of noise and vibration in rotorcraft."
Here is a bit on BVI:
http://www.aero.gla.ac.uk/Research/L...ero/Oldbvi.htm
http://halfdome.arc.nasa.gov/research/bvi.html