It makes no sense to step in on one of these situations. Arriving cops have no idea that you are helping and could well take serious action against you before they find out.
You have obviously never been in such a situation, and are again speaking in the Color of Saracasm I can only assume..
If the Perp were to get a hold of Officers Gun, a lot of really bad isht can then happen ..
If you held the perp down , The Officer would SIGNAL the other LE that you assisted, nothing bad would happen to you... .... but again, you speak like you know whats up and you clearly dont have a clue
.. Good People help HOLD down and Restrain violent Perps all the time .