You will be running rich when you come back on the throttle as the ECU will be expecting the air it has already measured coming in through the MAF. If you dump it out into the atmosphere, that air isn't there anymore, but the ECU thinks it is. You'll get momentary overfuelling, black smoke out of the exhaust and lower fuel economy.
Don't think you will do any permanent damage, but you certainly won't benefit from it (other than having chavs in McD's carpark looking over at you in awe).