While Knickebein was used by German crews en masse, X-Gerät use was limited to specially trained pathfinder crews. Special receivers were mounted in He 111s, with a radio mast on the bomber's fuselage. The system worked on a higher frequency (66–77 MHz, compared to Knickebein's 30–33 MHz). Transmitters on the ground sent pulses at a rate of 180 per minute. X-Gerät received and analysed the pulses, giving the pilot both visual and aural "on course" signals. Three beams intersected the beam along the He 111's flight path. The first cross-beam acted as a warning for the bomb-aimer to start the bombing-clock which he would activate only when the second cross-beam was reached. When the third cross-beam was reached the bomb aimer activated a third trigger, which stopped the first hand of the equipment's clock, with the second hand continuing. When the second hand re-aligned with the first, the bombs were released. The clock's timing mechanism was co-ordinated with the distances of the intersecting beams from the target so the target was directly below when the bomb release occurred.
When the bomber crossed the first beam the bomber-aimer did what?
start the bombing-clock