I like Makwa's summary.
Harrington on Hold'em Volume II covers M nicely.
An M under 10 means you're in trouble so you need to be careful about what you play and against who. Drawing
hands become less playable so forget about the middle suited connectors and small pairs.
When you get to an M of about 5, you're looking to shove with a *huge* range of hands into any unchallenged pot. The reason 5 is the magic number is because you still have enough chips to hurt people (fold
equity) and doubling up will be significant should you get called and suck out on them. If you wait until you have an M of 3 to shove not only are you more likely to get called, but doubling up will not improve your situation much.
Now, to complicate things a little more, you have to be careful against who you're making these shoves against. Shoving when middle stacks have the blinds are ideal, as you have enough chips to hurt them, so they're only going to call with nice hands (which isn't likely). Big stacks are apt to look you up with less because they can afford it, and short stacks are apt to pick that time to make a stand out of desperation.