I have a book of Geoff Hearn where head weight subject is briefly described and a DVD named Highlighting Head Weight. Actually never checked them until now, since I'm rarely dancing ballroom now. I'm not sure that I'm getting correctly what's in the book, because typical ballroom terminology is used, but from what I understand and from what's demonstrated on the video, head weight is meant as integral part of shaping/sways that are used in b/room standard. Mostly role of the lady is described - she should turn her head (weight) naturally according to the shape given by the man, so if it is sway/shape to left, head turns to left, if it is to right, head turns to right. Similar rule applies to the man, but there is much less shaping, hence much less head movement for the man. Anyway, as a general rule, line of the head should follow the line of the spine, so no broken necks and similar things
It's somewhat similar to rules for the shaping in zouk - actually exercise given in the DVD is similar to some exercises we did in zouk classes.
However, all these things are connected with swinging actions used in dances like ballroom standard. In salsa and ballroom latin, we don't have that, but we have cuban motion, which is a different kind of body movement, where head is relatively statical, so these things are not really applicable. So in CBL for instance, chest and hips (depending on the standing leg on particular beat) should be parts of the body of leader and follower that go further apart to create tension and not head
Anyway, I also sometimes play with swing-style shaping in salsa, for instance when leading backspot turn I like to make some shape to the right, changing to the left when lady passes to my right side. I've seen some other (non-ballroom) guys doing that and I liked it, although it is not a kind of cuban motion I suppose ...but considering that there are some elements of WCS and hustle in xbody salsa, I suppose that some amount of these things can be used from time to time, depending on the music and the partner ... some girls like this and some don't. However, it's always shaping of the whole body and not throwing just the head into some direction, just like cuban motion isn't throwing hips into some direction like some believe it is - both look weird