[raffriff42]

Sure, it's in the source code:
http://avisynth2.cvs.sourceforge.net...ew=markup#l815

	if (dropframe) {
	  if (rate == 30) {
		int c = 0;
		c = off_min + 60*off_hour;  // number of drop events
		c -= c/10; // less non-drop events on 10 minutes
		c *=2; // drop 2 frames per drop event
		offset_f -= c;
	  }
	  else {
//  Need to cogitate with the guys about this
//  gotta drop 86.3 counts per hour. So until
//  a proper formula is found, just wing it!
		offset_f -= 2 * ((offset_f+1001)/2002);
	  }