Emily Foster of Ten Six Consulting often writes excellent project management articles. The new one is titled The Negatives of Negative Lag. It explores the risks involved in using negative lags (aka “leads”).
One little addition is to note that in positive lags on SS (and SF) relationships, the lag almost always represents what Spider Project would refer to as a “volume” lag, which represents work/time that is already modeled in the predecessor, i.e., digging the first 25m. of trench. If it is on the CP, the drag and drag cost would then be in the predecessor activity, pointing the scheduler to the fact that to shorten the project, we need to get that first section of trench dug faster. (BTW, a lead, being negative time, has no drag.)
There are often good articles at the Ten Six Consulting site. I find it a useful bookmark.
Fraternally in project management,
Steve the Bajan