Junior Member, warming up (<31 posts)
Join Date: Mar 2010
Location: New York
Thanked 16 Times in 6 Posts
They are just referring to using a device to help the car maintain power while you change a battery, as many cars lose various settings (radio stations, time, etc) when you disconnect them from power. On the M5, using the steps I outlined in the OP, my experience was that I only had to reset my date and time - nothing else was lost, and I did not use any sort of memory saver.
Note that a memory saver device is essentially like a small battery that you plug in to your cigarette lighter (or similar outlet in your car). It provides just enough power to your car's computers while you disconnect the main battery so that the computers never fully lose power and are able to maintain certain settings that might otherwise be lost. Whether settings will actually be lost by disconnecting all power is more a function of how the car stores the settings, and every car is different. On the M5, I believe the cigarette outlet is not powered on when the car is turned off anyway - meaning a memory saver shouldn't work in the first place in our car.