THIS IS NOT FINAL... feel free to edit and add suggestions
The Low 2 High Auto Callback is an old protocol extension used by some eMule mods and the eMulePlus client. Because of the new Mod Prot it requires an update. The main idea is simple: when a LowID source has a HighID client in his queue he will establish a connection to reask the HighID on its own and the HighID will use it instead of triggering server/kad/xs based callbacks. That way, callback overhead is reduced.
A mod announces the support for this feature in the common mod info tag (1 bit), if both clients have set the bit than the HighID will not trigger any callbacks unless 59 minutes passed by since the last connection he had to the LowID, the LowID should automatically callback in 29 (normal) or 58 (nnp) minute intervals, the exact callback interval can be chosen freely by the LowID from 29 to 58 minutes.