Remove PCV hose & Cap it off.

A crankcase ventilation system is a one way passage for gases to escape in a controlled manner from the crankcase of an internal combustion engine.

This is necessary because internal combustion inevitably involves a small but continual amount of blow-by, which occurs when some of the gases from the combustion leak past the piston rings (that is, blow by them) to end up inside the crankcase, causing pressure to build up in the crank case. For control of the pressure inside it, a PCV is used to vent the crank case

 

For this trick to improve millage there would have to be allot of blow by. This guy's engine is probably hosed.

 

The claim is just another Internet myth.
Unless you block off BOTH openings that are created you will either be creating a vacuum leak and making the engine run lean OR creating an open vent for the crankcase that will blow fumes out and suck dirty air IN.
Also it is an illegal modification of the emission systems.

 

will this affect emissions?

 

Yes it will probably have very small bad effect on emissions. But more importantly it’s going to reduce engine life and make it more likely to leak oil.

This modification leaves original crankcase air inlet connected to clean air after air filter. And it will leave original crankcase air outlet tube just free. Since engine air cleaner will have some air resistance it will create pressure drop so now air in crankcase is going to flow in wrong direction. Air will get into crankcase from that unconnected hose sucking dirty air in and that dirty air will exit into air that should be clean. And after this modification crankcase isn’t going to have vacuum so seal life is going to be reduced. Also air flow rate in crankcase is going to be way slower than originally.

And if you would block that hose that was just left unconnected in that video then air flow rate thought in crankcase would be zero. And that wouldn’t be good for engine either.