Plot the voltages on the two transistor bases. They show identical triangle waves (imagine these as noise) imposed on sine waves (representing desired signal). Now cancel those plots and plot the current through the 1k resistor. The noise has been stripped off and only the signal remains. The circuit rejects the in-phase noise and amplifies only the sine waves which are 180 degrees out of phase. NB: There is an auto-ranging glitch which is a bug in EveryCircuit.
