Complex sentence: For the next impulse the threshold is crossed immediately before the trigger edge and so the comparator is only briefly positive.Vin (a) goes to full scale, + Vref, shortly before the end of the next impulse.
Split into simpler sentences: For the next impulse the threshold is crossed immediately before the trigger edge and so the comparator is only briefly positive. V in (a) goes to full scale, + V ref, shortly before the end of the next impulse.