Most effective pedal position to jump start from still situation
When you are starting, most force comes from your weight on the front pedal. The highest leverage is available when the pedal is in the 3 o'clock position. But starting there would waste the (potential) energy from going from the 12 o'clock (no leverage but maximal potential energy) to the 3 o'clock position. Hence, it makes a lot of sense starting with pedals in the 1 o'clock position with leverage increasing towards 3 o'clock and decreasing towards 6 o'clock.

A physics student could come up with an analysis for the best starting point by also taking into account the inertia of the system but I can't and have to leave it to may hand-waving arguments above.