The -10c mod makes your electric fan come on at a lower temp (-10c). The change is not limited to -10; that is just a common amount to change it (hence the name).

This is popular with some E55 guys and presumably this change can be made on any MBZ (and SRT-6?) in the "correction programming" of the motor electronics (ME) section.

debatable benefits, essentially keeps fan on most of the time once the engine has warmed up (since the fan switching point is changed independent of the thermostat operation temp).

Here is a link to doing it on an E55 with screen shots .... post #25 (note the caption for the last screen shot is wrong in that it suggests putting in a negative value)
Star Diagnostic How To DIY with lots o pics - MBWorld.org Forums

Some really like this, but I have had a Star for years and have never bothered with this on any of my AMGs

Cheers,
Chris