*There is no Creator other than Allāh*
It was said:
"That is, there is no Creator that exists other than Allāh Most High. He is the Creator of everything, which includes the actions of the servant, who has no ability to create. However, the servant does have the ability to choose his actions, as he has "free will" (i.e. Allāh has given him the ability to choose what to do, but even this granting of ability for him to be able to choose what to do in this life is granted to him by Allāh). Based on that choice then, Allāh creates the action, if He Most High wills for it to happen. Yet because the servant himself decided to do it, be is said to have ACQUIRED that action. Hence, the servant is judged by his "acquisition" while the act of "creating" belongs to Allāh alone."
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This means that if a man decided to stab someone, the man himself does not have any independent power and neither does Allāh give him share of His power to create, rather when he stabs the person, it is Allāh who creates the movement of the knife, it is Allāh who creates the piercing of the knife into the skin, only if Allāh wills for it to happen. If Allāh never willed for the knife to penetrate his skin, no matter how many times the man lunges into the skin, it won't penetrate. It must also be known that even when a slave wills to do something, he can only will what Allāh has willed for him.
The belief of the People of Sunna is that we don't have independent power or any share in power to create as we are alive and neither do we have this when we are in the grave. We always believe we are dependent upon Allāh and Allāh is ONLY Creator.
If you believe that humans whilst alive did actually have some intrinsic ability themselves to do something or create themselves or haqiqi (real) power to do something given by Allāh, then it's only natural that you will reach the conclusion, once the human dies, these powers are taken away from him, and he becomes absolutely terminated.
The People of Sunna never even believe we had any intrinsic or independent ability or haqiqi power to do anything ourselves whilst we were alive even in the first place. Our power to do anything is only majazi (metaphorical) It's always Allāh who creates everything. Thus we don't believe these "powers" leave us once we are dead. We never had any independence to begin with. Thus we can believe Allāh can will for those in the grave to help us through prayer because Allāh creates this and the pious is always dependent upon Allāh.