some bongs are built very inefficient.
for example:
when you fill the smoking chamber with smoke by sucking, youll want to exhale that 'empty' portion before going on to inhaling the column of smoke..

as well as this, you shouldnt over-inhale, even if theres a tiny bit of smoke left, the amount of excess air you breath in makes it pointless.

when you figure out how to get a neat column of smoke in you without diluting it with too much air, and manage to hold it for at least 10 seconds. youll only need one bowl

gravity bongs [2 litre bottle with bottom cut off, submersed in water] use this principle, since you cant suck in anymore air before or after the smokes gone

i watched this video 3 or 4 years ago, built a simmilar one and never went back [well, till i figured out how to use a bong more efficiently, even then it doesnt come close to the gravity bong]

Not your standard gravity bong