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: Q76Heart Suit54Diamond Suit108765club suit653, 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-2Heart Suit) 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 2Heart Suit (likely making). Similarly, if responder to 1 holds: Q 8 7 6Heart Suit6 2 Diamond Suit6 4club suitK 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-3Heart Suit-3). With 5 trumps, responder to 1 will raise to 4 (which is defined as 5 trumps and weakish). Example: KJ876Heart Suit5Diamond Suit8432club suitQ32. 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.