anyone know how SourceTime() does work?

The expression sourceTime() returns the current time from the in point of a footage layer. You can use it to make something happen after a certain amount of time. For example, this expression starts rotating at 100º per second 1.5 seconds after the in point. If the layer is movie footage with timecode, sourceTime() returns the timecode converted to the seconds. Another idiosyncrasy is that you can open footage interpretation in the Project panel and override any embedded timecode. 

st = sourceTime(time) - 1.5;
	if (st < 0){
		r = 0;
	}
	else{
		st * 100
	}

i usually use inPoint instead of source time because that starts the counter where you set an in point (Alt/Option + [) instead of the start of the original source.  I hope that makes sense. If the layer has timecode, unless you override the original timecode using the Interpret Footage, sourceTime() returns the original timecode converted to seconds.