How did this F/A-18's execute a "low-boom maneuver"?
I did some reading and this is what I could find out so far:
https://www.nasa.gov/aero/nasa-prepares-to-go-public-with-quiet-supersonic-tech
This article is similar to the Engagdet one but goes into slightly more detail(though not enough for a proper explanation) about the dive maneuver.
However, Lockheed Martin is in the process of designing and building a low sonic boom demonstrator, slated to fly in 2021. The wikipedia page says that to prevent sonic booms from building up, the aircraft will have a long slender body and canards (thought the Concorde had these as well, so I'm not sure what the difference is). They will probably add canards to the F/A-18.
The dive maneuver happens in such a way that while the sonic booms directly underneath the aircraft are strengthened, there is a secondary pair of sonic booms created further ahead that are weaker, giving quiet thumps instead.
Hope this helps.