Harrington covers this well in Harrington on Hold'em Volume II.
What you're asking about is an inflection point.
First a little lesson on M. Your "M" is the number of rounds you can survive a the current blind level. Let's say it's late in a 90 player S&G with $10 antes and $100/$200 blinds, so over nine hands (one rotation at a full table) you'll put in $390 per round. So in that situation if you had $4,000 in chips your M would be right about at 10.
Your M dictates what moves you can make effectively. An M of 20+ and you're pretty much in good shape and can play your normal game.
An M of 10-20 and you need to be a little cautious. Drawing hands aren't quite as valuable, so be a little pickier with them.
An M of 6-10, you really need to tighten up. Drawing hands aren't usually going to get the payoff they need to be played profitably.
Once your M gets to 5 or 6, you are in shove or fold mode. The reasoning behind this is because you still have enough chips to hurt most callers, so you still have fold
equity (better hands like K/T to your J/9) will fold. Also, taking the blinds is a significant win for your stack.
Even getting called by a better hand is not tragic, as folding a few more rounds will leave you with the same small stack if you double up. (IE, shoving when your M is 3 means you're still in trouble at 6). Getting called with a better hand usually means you'll still win a little more than 1/3 of the time, and when you do win you will have a decent stack again.
So to answer your question, you play your normal game and bluff when you see the standard opportunities. As your stack whittles down bluff less, but when it gets really low you bluff more.