Redlining is pushing an engine to its maximum safe operating speed. Most mechanical engineering types and roleplaying gamers will probably know this definition, but it's a little bit specialised and there are other definitions of "redlining" in fields such as loan accounting, document editing, and possibly others. So I thought a little explanatory note here might help, in case of confusion.

And if you were confused, you've now learnt something! Isn't education fun?