since the secondary are of identical number of turns they will produce identical signal but with a phase difference of 180 deg with respect to center tap. When one end is(say Q1 base end) positive going, the other end Q2 Base end) will be negative going. As Q1 is getting positive going signal it will conduct but Q2 cannot conduct as the input is negative going. In the next half cycle Q1 input will be negative going hence it cannot conduct. but at Q2 input will be positive going hence it will conduct. This process continues. Since Q1 & Q2 are conducting alternately it is called push pull output circuit.