I've heard of that trick before (using a bar against the floor and tapping the starter). I would never do it myself. I just don't like the idea of:
1) somehow doing something dumb and spinning it in the wrong direction - imagine that breaker bar flying into the radiator and front engine accessories.
2) subjecting the crank to a torsional load along its entire length that it wasn't designed for.
3) breaking a tooth on the ring gear, warping the flexplate - who knows what else.
I have a bunch of 1/8 x 4" steel plate that is 6 or 8 feet long. Every time I need to take off one of these stubborn crank bolts I build a tool to hold the crank in place. The hub of the front pully has bolt holes to mount a puller. Just build a tool that bolts to these holes, cut a hole in the middle for your socket and then use a big breaker bar with a cheater bar (pipe) to get it loose.
I personally would rather use this approach than crossing my fingers, hitting the key and hoping that I don't have a bunch of broken parts a split second later.
I don't use an impact driver on those bolts because booger-ing up threads in the crank snout would be terrible.
Experience is something you don't get until just after you need it.