In other words a negative delay means the number of seconds you are willing to listen to a channel before going on. So a -5 delay means it will listen to the channel for 5 seconds and the resume scanning. (If there is something "juicy" going on, you can hit the Channel button and hold on the channel).
In general, I do not use negative delay. However, one exception is Dallas Fire Rescue's automated Locution dispatch. Every now & then, 'Dispatch Debbie' goes off the deep end, and keeps dis[atching the same incident, with the same units assigned, multiple times. So, when she goes bonkers, negative delay tells the scanner to go find something else until 'Debbie' gets laryngitis.
I use negative delay with unattended search. When the scanner stops on a carrier it would be stuck there but with 10 sec negative delay it will record what is there and continue the search. I have to use large search ranges to have a cycle time of one minute so that a 10 sec stop on a carrier doesn't take up the majority of the search time.