The heart is an aggro meter directly on top of titan's aggro. If you have more aggro on the heart and titan's main body combined, then you will take aggro. Nothing more to it. If a tank lost aggro during the heart, it's because he didn't have enough aggro during the main boss phase. Alternatively worded: The heart is an extension of titan's aggro meter; so what ever you had before the heart phase began is the base value when it begins.