Pulse Width Modulation is often used as a method for charging lead-acid
batteries. It greatly increases battery life by reducing or eliminating
sulfation, as described. I am assuming this is what they are doing.
My own intuitive sense of it is that the entire plate dissipates the
variable width pulse at a slightly higher voltage than would otherwise
occur in a trickling scenario, whereby the current favors the lowest
impedence routes to the electrolyte.
PWM chargers are also widely used with deep cycle batteries and
wind generators or photovoltaic arrays for home power applications.
-Doug