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...

But why providing a soft start for a load which easily withstands normal start?? If you go into detail with your approach, you will very quickly notice, that

1. an enormous amount of axtra costs will result, and

2. that now an over-stress condition somewhere else must be kept under control!

So, why not stressing the part, that is designed to withstand the hard turn-on? And if a soft start is needed a very specific solution is used anyway (soft start for transformers, pre-heating for big lamps...)

Kai