Invitation to arrest an outpouring (3,4)
I believe the answer is:
say when
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for say when that I've seen before include "Words from the pourer?" , "Tell me when to stop pouring" , "Request to be told to stop pouring drink" , "Pourer's words" .)