Just what we need -- another rule! Actually, this one is a bit archaic, but has some merit. It concerns preempting and vulnerability.
Unfavorable - "2"
I am a big believer in observing the vulnerability when preempting. If "Unfavorable" (Vulnerable against Not Vulnerable), I recommend being sound and cautious. The Rule of 2,3,4 states that when at this unfavorable vulnerability, you should be within 2 tricks of your bid. So, let's say you have this hand,