If used to steal a vampire in combat, combat ends. Any damage yet to resolve still resolves before the change of control.

If used to steal a minion attempting to block, if the block attempt becomes illegal, the block attempt fails.

If used to steal a minion during an action, Methuselahs who declined to block are still declining to block. The new controller, if he is eligible and has not declined to block yet, can block with the stolen minion even if the previous controller has declined to block.

If used to steal a minion targeted by an action, the action continues with the same target. Methuselahs who declined to block do not get another opportunity. If the target is not legal anymore (eg. D Slaughtering the Herd), the action fizzles (ends successfully with no effect).

Can be used to take control of the target at any point during the appropriate minion phase. It does not have to be done before non-mandatory actions.