Assuming constant and well formed sail twist, each sail is designed to have a specific angle of attack when mounted either to a boom or to a tack—fairlead car angle (about .... typically close to 10° -12° off of the boats centerline. Bringing the boom 'across' the centerline to the windward side will 'put on the brakes' and cause massive separation stall on the leeward side --- a parachute!!!! If you want to try this, just watch to see what happens to the tell tales, and that alone should tell you that this 'technique' will be a disaster - the leech and perhaps the midcord tell tales will be pointing FORWARDS on the lee side. This is what you'd expect from a parachute; and, parachutes dont fly well 'to weather'.
There is ONE time when racing and in light winds when one 'may' slowly and for short duration, slowly pull the boom across the centerline ---- to send massive turbulence back to a closehauled and overtaking-from-the-stern competitor. The competitor should totally lose flow attachment, sail into a large 'header' due to the massive rotating turbulence being 'sent' to him/her; and, it will take some time for his 'flow' to realign, etc. --- it will be like he/she just fell into a hole in the water and will have to radically bear away on the 'rotor' that youve 'sent' him. The caution is however that you may cause your own boat to come to a stop or severely slow down. Better to do this by power pinching as described in the next paragraph as it sends a more compact 'rotor' to a competitor on your stern.
Power pinching or 'putting the flaps down' for brief periods involves pulling LOTS of strain on the mainsheet when closehauled to deform (hook-up) the leech. This will cause the camber or amount of draft of the sail to radically increase - similar to a larger aircraft with its 'flaps down'. This is an almost standard technique by top class racing sailors near the end of a long closehauled beat ... put the flaps down just several boat lengths before tacking (where sailing a strict VMG course is no longer a benefit) with the result of saving 'several' boatlengths of distance on the 'next' leg because the boat is now radically 'climbing to weather'.
Racing is a game on 'inches' and what you save on the 'next' leg by 'hooking up the leech' at the end of a windward leg can be substantial even that the boat is now slowing down. The 'best' can usually save one or two boatlengths or more per 'leg' over someone simply straight line beating then tacking over; but, dont 'overdo' the leech hooking.
However if you bring the boom over to the windward side to do this ... enjoy being passed by just about every boat in your fleet. ;-(
