I think it's important to distinguish a procedural vote versus crafting actual legislation.
What this procedure does (as best as I can understand arcane legislative processes) is allow a late and final chance for the minority to insert amendments to the bill just before voting.
The reason this is a danger to the majority is it allows the minority a last second Hail Mary to insert cleverly crafted poison pill amendments or other verbage gutting the bill. A last line ".... NOT! LOLOLOL"
This is after the committee process, after all the negotiation, compromise, deal making and vote whipping.
In this case, an amendment regarding reporting illegal immigrants to ice was inserted into a gun control bill by the defection of 26 Ds. Something unrelated to the original bill.
The 26 defectors, scared of some negative vote scoring, then allowed in a troll amendment that forced THE 200+ OTHER Ds OF THE CAUCUS to vote FOR the troll, else vote AGAINST their signature gun control bill.
Because of the fear of a few members on a bullshit procedure vote, they ratfucked the rest of the caucus to take a tough vote for ice or against gun control.
No wonder ppl are pissed and making threats.
Pelosi and AOC are saying "you may fear the activist group vote scorers, but you better fear the rest of us more. You can beat them, but we will guarantee your road to ruin."