This is a formula for deciding when to make a penalty double (and for deciding when to pass a takeout double, converting it to penalties) when the HCP are relatively evenly divided.
Add (1) the level of the contract plus (2) your number of trumps plus (3) your number of honors in the trump suit; if the total is nine or more, this formula suggests that penalize the opponents.
As an example, say that the opponents are in one diamond, and your diamonds are AJ9743. Here, you would add 1 (the level of the contract) plus 6 (your number of diamonds) plus 2 (you have two of the five diamond honors) = 9, indicating that you should penalize the opponents.
For more information, see Chapter 5 of Real World Bridge 5: Competitive Bidding Part Two.