This is brief because I've got an impetus of my own and need to master it.
1. a force that moves something along [syn: drift]
2. the act of applying force suddenly; "the impulse knocked him over"
Impetus is what propels you, when you get that feeling that blows you away, when the plot problem finally makes sense and makes great art, the song that brings dance back, the ambition bright in the early morning when the world is possible.
Don't wait for the impetus to write, because you won't write much. But when you get it-- some way, some how-- take it and run.