Normally, when Maemo Mapper tries to draw a map, if it can't find the map (i.e. it hasn't been downloaded), it tries the next higher zoom level and, if it finds it, scales it (thus the "Double Pixels" effect). If it doesn't find a map at the next higher zoom level, it tries again at the next higher zoom level, etc. So, in your case, when you zoom in to level 0, since you don't have level-0 maps, Maemo Mapper draws the level-1 maps instead.

The "Double Pixels" feature forces Maemo Mapper to exhibit the above behavior at least once, even if a map exists at the current zoom level. So, if you had level-0 and level-1 maps, and if you are at zoom level 0, then it will draw the level-1 maps. If you only have level-0 and level-2 maps, and if you are at zoom level 0, then it will draw level-2 maps. If you are always in Double Pixels mode, it is worthless to download level-0 maps.

The Double Pixels feature can be applied on a per-repository basis, so if you have a map source with fonts that are just too tiny to read, you can use it just for that repository.