On user request or other arbitrary external event, whenever we want to turn the power on. It doesn't care where the AC phase is at the moment at all, it just decreases the blocking ("not triac") device resistance gradually so the current rush never occurs - even if the target receiver resistance is very low at first, the "dimmer" switch makes up for that with high resistance in the early phase. The result is that first few cycles are out of spec, i.e. (like on the picture) could be easily considered frequency higher and amplitude lower than specs say.

Now the question remains - if not triac, then what? Two big MOSFETs in two directions? An optocoupler with 5V:230V ratings? A potentiometer with some kind of drive?