In most modern designs, you have to work with ICs that are running at very high speeds. This translates into the I/O pins of an IC going from logic state zero to logic state one in a very short amount of time. For an IC to be able to function that fast, you need a power source placed really close to it so that the appropriate amount of charge can be dissipated in a short amount of time.
The VRM and the bulk capacitors near the VRM have large amounts of charge but the speed of delivery of the charge is fairly slow. A decoupling capacitor (decap) stores and dissipates the charge, thus making it essential to the working of the ICs on an actual printed circuit board. If this decoupling capacitor is placed too far away, the inductance of the trace and the pins can make the capacitor ineffective.
With HyperLynx DRC you can run rules that will check to see whether the decoupling capacitor is placed too far away from the IC power pins. Simply enter the acceptable radius of search from the IC pin and HyperLynx DRC will highlight all the nets that violate this condition.