Horizontal switch:
>At the center of the picture, current is 0 and voltage is 0 and the transistor is on.
>Current ramps up to 6A, V is close to 0 because the transistor is on.
>The transistor is told to turn off. There is a large capacitor across the C-E of the transistor so the voltage can not rise fast. In the time the transistor turns off (current falls from 6 to 0A) the voltage only rises maybe only 10 volts. ZVS
>Now the switch is off and the C across the transistor and the Ls in the system resonate and cause a 1/2 sign wave of 1000 volts.
>At the end of the 1/2 sign the voltage is back to 0. The current drives into a diode across the transistor. (the voltage is 1 diode drop negative and current is in the diode)
>The transistor is turned on with a negative voltage on CE and 0 current. ZVS and ZCS
>Current ramps form -6A down to 0. {repeat the sequence }
With out ZVS it is possible to have 3A and 500V across the transistor for 100nS. Huge power loss.