At +6 BAB, rather than a single attack on a full attack, one can make two attacks, both at -3 to attacks (+3/+3). At +11 BAB, this penalty is reduced by one to -2 (+9/+9), and it is reduced by one again at +16 BAB. All attacks during your turn have this penalty, even if more are gained (for example, from Flurry of Blows).
The use of this variant actually gives a small boost in power to meleers who need to roll around 8-13 to hit an enemy at their highest attack bonus. Only when the AC of an enemy is very low (required 1-3 rolled at highest BAB) or very high (requiring 18-20 rolled at highest BAB) will with the "usual" method of iterative attacks be more effective.