Simple,
The DC-DC converter IC is switch on/off and the output waveform is like ac voltage, but rather than alternate +/-. Its alternate Gnd and V+.
But since there is an inductor that cannot stop current flow immediately, the circuit can boost voltage greater than Vin, and same with the negative cycle, that can lower than GND..
the diode determine the positive/negative voltage that can be taken to the circuit...