- I just slapped the phone out of their hand / threw them against the pavement / committed some other sort of assault, usually because they somehow affronted my honor (eg, by asking what law they were breaking).
- If I just walk away, it's clear that that was unjustified.
- So I fabricate a resisting arrest / obstructing an investigation / disturbing the peace / assault on an officer claim for them to fight, so it devolves into he said / she said, where I win by virtue of my reputation as a cop & the lack of AV evidence.
That's what the "we have to cover our ass" discussion is about.