Please check out this thread -> https://www.hdforums.com/m_1654514/tm.htm

 

The Threoy is correct...hot exhaust gasses travel around 280 to 300 fps at this speed the gasses can't make a 90 degree turn and go out the left pipe. However the gasses are forced out the left pipe at higher throttle openings, It is true that the blow down of the rear cylinder will suck air through the left pipe....the more over lap a cam has the worse this becomes but it doesn't effect the AFR in the combustion chamber makin a lean condition but it does make it tough to tune the rear cylinder. The trick to tuning is to get the sniffer probe past the left pipe opening and close the the exhaust valve as possible...if you don't do this or block of the reversion from the left pipe the rear cylinder tune will be off. Bottom line a hp drop will happen with the beer bottle stopper in while you ride....it's like the banana in the tail pipe trick.
Good Luck