Why In a center-tapped full-wave rectifier, the transformer secondary's center tap is the return (ground) and each end of the winding feeds one diode. The two ends swing opposite in phase, so one diode conducts on each half of the AC cycle, producing two output pulses per cycle at twice the line frequency. Only two diodes are needed because the center tap provides the return path, but each half-winding only supplies half the total secondary voltage.
Watch out A full-wave bridge also gives two pulses per cycle but uses four diodes and needs no center tap; a half-wave circuit uses a single diode and conducts on only one half of each cycle.
Two diodes plus a center tap equals full-wave; four diodes with no tap equals bridge; one diode equals half-wave.