They're angry or sad or both and have been triggered so go to a bridge to contemplate suicide. Or look like they're contemplating suicide. Or self-harm at home and concerned family members call the police. Or they have an argument at home, get dumped by a partner or some other trigger and threaten to kill their Mum/Gran/partner. Or smash things up, break windows, shout at or threaten people at home or in publlic and generally scare people so the Police are called.
Emotional dysregulation literally means an inability to control their emotions so having an extreme reaction that is outside of the 'norm'.
So The Police are called and if outside the home, the individual is sectioned by the Police on sec 136 of the mental health act so the person is taken to a place of safety to be assessed by MH professionals. Who will request a mental health act assessment if they feel the person meets the criteria to be detained in hospital. Or the person might be arrested by Police for breach of the peace, assault etc but then referred to custody mental health professionals due to their concerning behaviour in cells.
In the case of the met police 78% of the people they were detaining on sec 136 were not detained under the MHA and released. Some may be referred to a community mental health team for follow-up. Some will be released with information about how to access counselling or other resources which may be helpful.
Some will engage with a CMHT if referred, some will not. Some will follow up on resources given, some will not.
Many people are repeatedly sectioned by Police, some more than once in a night. Some several times a month. Some more occasionally but still a pattern.
They don't meet the criteria to be detained to hospital but also don't want to/are unable for whatever reason to access the voluntary resources that might help them so it goes on and on and round and round.