Every now and then, you are not quite sure whether to bid or even bid one level higher than the opponents. Every now and then, you are not quite sure whether to sacrifice to obtain a better result than the previous table. Every now and then, you are not quite sure whether to preempt, attempting an Advanced Save.
The Rule of Two and Three is sort of a guideline used in determining whether to make a preemptive bid, whether to overcall or overbid, whether to make that sacrifice, or simply pass.
Using the numbers, a bridge player should be aware of the fact that he cannot afford to be set more than 500 points. This number is rather the magic boundary. If the opponents have game and they are not vulnerable, then the result equals minus 400-420 points for you. If the opponents are vulnerable, then the amount equals minus 600-620 points for you.
If you are not vulnerable, doubled, down two, then the result is minus 300 points for you.
If you are vulnerable, doubled, down two, then the result is minus 500 points.
But, if you are not vulnerable, doubled, down three, then the result is minus 500 points for you.
If you are vulnerable, doubled, down three, then the result is minus 800 points.
Conclusion: taking the risk of being down two tricks, doubled, vulnerable, or taking the risk of being down three tricks, doubled, not vulnerable, is not all that bad.
The guideline passes along the information that at a contract of 4 Hearts, the declarer should have 8 tricks if he is vulnerable, if this is clearly a preempt or sacrifice, and 7 tricks if he is not vulnerable. The position at the table should also be taken into consideration
With the Rule of Two and Three, there is very little mathematical calculation besides adding up quickly the different results very likely achieved according to vulnerability. As a bridge player, you are looking to achieve the best score, but there should be reasoning and logic and information behind your Preempt, Sacrifice, Advanced Save or Overcall.
Please remember that the Rule of Two and Three applies only to the almost certain assumption that the opponents can bid game, not slam. The action of sacrificing against almost certain slams takes on another form.