Source:

The LAW of Total Tricks (LOTT) is a bidding guideline developed by Jean-Rene Vernes. It helps you to decide how high to compete, using this approximation:
With 8 partnership trumps, bid to the 2-level.
With 9 trumps–3 level.
With 10 trumps–4 level.
So, if responder to 1

holds:
Q76
54
108765
653, and hears partner open 1

, he knows his side has 8 trumps. He won’t raise immediately to 2

, because that would be a lie. Bids still mean what they always meant and raising to 2

shows 6-10. So, responder has to pass, but he knows that if the opponents now balance (say 1

-P-P-2

) that he should be willing to compete to 2

since the partnership has 8 trumps. It doesn’t mean 2

will make, but even if it is down, that is likely better than defending against 2

(likely making).
Similarly, if responder to 1

holds:
Q 8 7 6
6 2
6 4
K 8 7 6 4, he knows his side has 9 trumps. He can’t raise to 3

since that would be invitational. But, later, if he has to, he should compete to the 3-level (1

-P-2

-P; P-3

-3

).
With 5 trumps, responder to 1

will raise to 4

(which is defined as 5 trumps and weakish). Example:
KJ876
5
8432
Q32.
Over partner’s 2-level preempt (6 cards), be willing to compete to the 3-level with 3-card support (9 total trumps) and the 4-level with 4-card support (10 total trumps). After a 3-level preempt by partner, assume a 7-card suit and use the LAW accordingly.
Use LOTT only when you don’t have game or slam interest.
Never Lie (1
-2
means what it has always meant).
Use the LAW mostly on the 2-3-4 levels (not on the 5-level or higher).
The “LAW” states that on most bridge deals the total number of trumps is approximately equal to the total number of tricks.