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The Spot Loader keeps the chunk it is in loaded into memory constantly, allowing for all the game's processes to run normally in that single chunk, regardless if a player is present. See http://www.minecraftwiki.net/wiki/Chunks for an overview of chunks.
A redstone signal will turn this block off. When off, the orb will come to a rest.
In contrast to the Chunk Loader, this block does not have a GUI and is not configurable.
The Spot Loader is most useful when you need single chunks loaded that are not near each other—for continuous chunks, it is more advisable to use the Chunk Loader for the ability to load many chunks with a single block.