The Comments plugin will broadcast plugins to the child blogs and, optionally, keep the comments of selected linked posts in sync with each other. The one-time sync is from the parent post to the children.
Syncing is done on a per-post basis. The direction is either from the parent to the children, or bidirectional. A sync is executed automatically when a comment is:
modified (approved, spammed, unapproved, etc)
To enable automatic syncing, edit the post and chose the direction which you want the comments to sync: either only from the parent post to the children, or bidirectional.