What’s not to understand? Their goal is the expression of power, the suspect’s escape is completely non optional to them. They are not trained in de-escalation.
So, by placing their body on the line (in the path of the giant metal machine), they are essentially saying “obey me, or you are willing to kill me if you try to escape”. So if you try to escape, you are willing to kill them. So if you are willing to kill them, they are free to defend themselves.
It is crazy, but consistent logic.
Remember they aren’t there to help, or protect, but to enforce.