Used this tool recently and think there is a purpose to help remove blockages but had most success when letting it mostly empty and closing off value to hose and turning the water back on it seem to stir up crystals better and removed about 2 pounds of sediment.  Live in Texas where shower in limestone.

Some thoughts about water heaters and your video for what they are worth.

1) You show turning the gas back on at the end but you should mention about turning it off at the beginning before starting the drain.

2) The problem you are having draining the tank is caused by a copy of things:
a) you are using the terrible cheap drain cock that came with the heater
b) you do not have a air vent piped into the system  so there is no easy way for air to enter the tank when drain cock is open.

3) when you get your next water heater I would suggest before filling it:
a) you purchase a pan to go underneath it which can be attached to a hose to drain any water that might start to leak out (some places this is required).
b) you purchase a battery water monitor and place it in the pan to detect any leaking water and produce an audible alarm.
c) you purchase and plumb in an air vent above the heater (faster draining)
d) you spin out and exchange the cheap plastic vendor supplied angled drain cock (which clog and break easily, do not always close tight when opened, and cannot be pushed straight through) and replace it with a straight threaded pipe with a good ball valve shutoff on the end.  You can add another short piece of pipe on the other end of the ball valve shutoff to screw on a hose to drain and a threaded cap when done (in case some kid opens the ball valve).

The ball valve shutoff has a larger opening (which allows faster draining), is faster to open (just 90 degree twist), and allows a straight through path for the opening.  This last part can come is handy when you open the ball valve and nothing comes out due to clogging of the opening.  You can stick/pound a screw driver straight through and bust up the clog and push it out of the way easily to drain the tank.

e) plumb in an appropriate sized water expansion tank above the heater (some places this is required).

When the heater burner is running and heating the water inside it causes the water in the system to expand.  That expanded water volume has to go somewhere.  Since you have a closed system when all drains are closed this air expansion tank takes up the expanded water volume.

If you lack an air expansion tank it put a much greater strain on internal plumbing over time and makes it more likely any pliable hoses used to connect devices might burst (causing massive water damage).

Since the heated expanded water volume has to go somewhere (as liquids cannot be compressed) for houses without expansion tanks this means water is actually pushed back into the home feed from the street as the pressure grows.

Many localities now require one way valves plumbed in where the water enters into your house.  This prevent any contaminated water in your home system from ever flowing back into the main water feed for the neighborhood (number on way local water is contaminated).  The trouble here is that these one way valves prevent any heated expanded water from leaving your house if you are missing an air expansion tank.  This can easily cause burst pipes over time.

For anyone with a water heater not in the basement you definitely do not water leaking from a burst or leaking water heater or burst plumbing from over stressed pipes or hoses.  This can cause extensive water damage if no one is home to shutoff the incoming water supply.  You can plumb in auto water shutoffs based on water detected on the floor or in a pan by the water heater.  They also have auto shutoffs that work on total water volume that try to determine burst pipes and shut off the entire house water supply but damage will have occurred already.


f) many localities also require water blending valves plumbed in directly above the heater.  These connect the hot and cold pipes entering the heater with a temperature valve in the middle.  If you set this to 120 degrees then any hot water entering the entire house system will never go above that temperature as cold water will be blended in immediately to prevent burns.

These water blending devices are important if you have children or elderly leaving at the residence.  Kids can get burned easily if the water heater is set too high and the elderly have less sensitively to notice it as well.

As an added advantage of using a blending device you can set your heater hotter which means your hot water lasts longer (use less) and you can plumb a hotter line to the washer (before blending device) for better cleaning.  You might be able to get away with a smaller heater if space is a problem by keeping the temperature higher (40 gallon instead of 50 gallon).

g) you might want to add a pressure and temperature gauge to the heater output line as well.  This makes it easy to see internal water pressure in the home and what temperature water is actually being produced.

Finally I see you have the emergency heater pressure flow plumbed into the wall.  If this ever opens it will release extremely hot water under pressure so it should only point downward and possible into a floor drain on the other side of the wall.