Stroboscopic Method of Slip Measurement

Hi,

In stroboscopic method of slip measurement for induction motors,

1. With combined ac and dc supply, stroboscopic lamp is supposed to flash only when the ac and dc voltages add, and remain extinguished when they oppose - why?

2. By counting the number of lines passing a fixed point in a say minute, and dividing by the no of lines seen (3 for a 6 pole), the apparent backward speed in rpm (slips speed) can be found. How is this justified?

Regards,