Plastic bag method that might actually be great

[BlissfulEnigma]

So, not long ago I posted about a potential method which involves hyperventilating followed by taping your mouth and nose shut. The hyperventilation part SHOULD lower CO2 enough to experience a hypoxic blackout, but I realized that 1. The tape might move around and let air in due to unconscious conclusions, and 2. I don't have strong tape around anyways. But what's it you used a plastic bag instead? So you hyperventilate until your CO2 levels are extremely low, then you place the bag over your head and tighten it as much as possible. It might be bit more complex than that, but how would this not be effective assuming the bag is airtight and for the most part airless?