Ahh -- that Idle status is not related to downloads. That just means LimeWire has noticed you're not using you're computer, so it disconnects from a few Ultrapeers, letting other users connect to the network easier. By releasing those Ultrapeers, LimeWire also needs less bandwidth so that more is available for downloads and/or uploads.

This is not a user configurable option because it is good for the network health. Allowing users to change the behaviour at will could be harmful.