The wave form is complex.
One end of the transformer has what you drew. (120hz half sign wave)
The other end is switches at 60khz or 120khz or what ever frequency the IC runs at.
>>When the IC pulls the transformer to ground (there will be the power line voltage across the transformer).
>>>>The power line voltage changes at 120hz. It will be any there from the very peak to zero volts.
>>Then the IC opens up the transformer will "fly up" above the power line voltage.
>>>>>The transformer will fly up to a level there power goes out the secondary.
>>Then three is likely a dead time there no voltage is across the transformer.
The voltage across the transformer will look like this: Frequency=100khz.
There is a line "Vin" that is "0" volts. When the IC is on the line voltage is across the transformer (negative voltage)
Black is typical.
Red is for then the line voltage is high.
Green and green are when the voltage is low.
For now don't think about the "leakage inductance ring", Just think of the voltage as the blue line. This is when power is sent to the load. The distance from "0" to blue will be the same at high Vin or low Vin.
The time after "core reset" where there is low frequency ring is call dead time. Nothing is happening. No current flows.