Detect Vertical Blank

Well, the flip() code is stored in BRL.mod -> graphics.mod, under line 341.

The code is as follows:

Function Flip( sync=-1 )
	RunHooks FlipHook,Null
	If sync<>-1
		_driver.Flip sync
		Return
	EndIf
	If _graphics<>_exGraphics Or Not _softSync
		Local sync=False
		If _gDepth sync=True
		_driver.Flip sync
		Return
	EndIf
	_syncTime:+_syncPeriod
	_syncAccum:+_syncFrac
	If _syncAccum>=_syncRate
		_syncAccum:-_syncRate
		_syncTime:+1
	EndIf
	Local dt=_syncTime-MilliSecs()
	If dt>0
		Delay dt
	Else
		_syncTime:-dt
	EndIf
	_driver.Flip False
End Function

I'll take a closer look and try to figure out what's going on here.