The technique on the "rotafixa" website is a good way to tighten a sprocket when you don't have a chain whip handy.

Now here's some physics: Applying sufficient back-pressure on the pedals will eventually unscrew the cog unless a lockring holds it in place. If the cog unscrews, any number of bad things can happen upto and including the destruction of you and/or your bike. Any further discussion of physics would involve estimating how much torque the cog sees in back-pressure braking senarios.

I suppose riding w/o a lockring is theoretically perfectly safe if you never apply back pressure on your pedals.