When writing with two or more people, and one person directs the other if they're having trouble with their plot.
Can also be used in the negative, like the backseat driver of writing.
Joe: I'm having trouble coming up with a reason for my character to want to go shopping today.
Larry: Oh, here, e-mail me what you have and I'll give you a plot-nudge.
Lisa: Yeah, whenever I write with her she always gives me way too many plot-nudges. She's practically writing for me!