For a minute Tuppence thought she was going to spring upon her, which would have placed the girl in an unpleasant dilemma, since she meant to draw the line at actually letting off the revolver.
If the girl kicks Tuppence, she will get shot. 
mnli hypothesis: If the girl kicks Tuppence, she will get shot.  premise: For a minute Tuppence thought she was going to spring upon her, which would have placed the girl in an unpleasant dilemma, since she meant to draw the line at actually letting off the revolver.