Greetings!
As far as my knowledge tells me, it is similar to a hot-swappable hard drive. The RAM will mirror its information (much like a RAID configuration), which will allow you to pull a DIMM without having to power down the system. You can pull and add RAM while in operation. Even with this feature, I would be reluctant to hot swap RAM though unless I knew for sure that everything was in order (proper type of RAM, proper setup in the BIOS, etc.). This is a very useful feature if you have a server that can not be turned off for any reason, but for most cases, it is really not a needed feature...nice to have, but really a special needs feature.
Hope this helps!