Dreamshell parameter SYNC/async

[sundance]

When trying to get a game working with Dreamshell,
it's necessary to modify a bunch of obscure parameters.
http://www.dreamshell.infos.st/ is a great source to consult which parameter must be used for which game,
although it's rather directed towards IDE-modified Dreamcast, not unmodified ones using an SD-Card reader on the Serial port,
which means that sometimes, the parameters suggested there are not applicable, or not the most suitable ones.

Anyway, just pushing some number into fields without understanding what's happening is unsatisfying,
and doesn't help to build the know-how in order to debug a situation.

Case in point : what is the SYNC parameter ? (named async in the preset text file)
I haven't found any documentation so far on this topic. Available values are : TRUE,1,2,3,4,5,6,7,8,16

What does it do ?
What is the benefit in increasing this value ?
What is the benefit in _decreasing_ this value ?
What are the consequences for memory usage ? speed ?

That would be extremely useful in understanding why some games require different values,
and which other values could be considered.