all we had to do was crank the flywheel over slowly (even taking 10 seconds from BDC to TDC.) and when it reached TDC the Impulse would occur, and the engine would thump into life.
On some early engines (WWI SE.5 fighter was an example) the pilot wound the handle of a booster magneto, which provided the necessary spark, and the stationary engine would burst into life. Should that fail it was then necessary to resort to hand swinging the prop.