The other one is the branch priority decision strategy:The branches with higher online stakes will definitely be heavier. Therefore, unless the historical stakes that an attacker possesses exceed all the online stakes of the current main branch, it will not succeed.  For example: the total stakes are made up of three equal part A，B and C. If you collect the private keys of part A and B at a history point, that's 2/3 of the total and you plan to build a new fork from the history point and replace the original one. But the owner of stake A and B must have moved their stakes to other accounts, like A', B', before they give the old private keys to you. So the original branch will have the active stakes of A'+B'+C and your branch will have the stakes of A+B. Your branch will never heavier than the original one under my strategy. Besides, your account A and B will become invalid when a new save point is established.