Breakpoints are points in game mechanics that the benefit of additional speed (in whatever process) is actually seen. For instance, having 85%FHR won't benefit a Barbarian any more than 48%, but getting 86% will allow one frame improvement in returning to battle after being hit for a set percentage of your life, which can mean increased chances of success and survival.
Whirlwind has a separate breakpoint system than other attacks. A link in the Barbarian information thread gives more detail, including which weapons can hit the last BP, and a weapon speed calculator should have the Whirl modified for .10.
