triacs from dishwasher
What you are describing is one of the methods I use to drive triacs.  The Triacs MT1 terminals are connected to the AC Mains HOT conductor and the load common leads go to the AC Mains Neutral conductor.

I create a negative DC supply voltage relative to the AC Mains HOT conductor.  That means that the micro Vdd pin is also connected to the AC Mains HOT conductor; the micro Vss pin is 5.1 Vdc below that rail (-5V supply).

That means that the micro output pins are HI when the triac is supposed to be OFF.  The micro output pins goes LO (-5V) when it's time to turn the triac ON.

Simple and effective.

[Edit]

It was mentioned that you are asking why this does not damage or kill the MCU.  The answer is that the microcontroller is floating with respect to Earth Ground.  Because it is floating, you can tie either the Vdd or Vss pins to whatever voltage level you want.  

In this case, we choose to tie the MCU Vdd pin to the AC Mains HOT conductor.

The only thing that matters is: Keep the voltage between Vss &amp; Vdd (and all the I/O pins) to the value that the manufacturer specifies -and- Stay within the current rating of the MCU pins.