first things first. in the u.s., each party writes its own rules for nominating candidates (and each state writes its own rules for how it's elections are run). this makes for a bizarre primary system where different states have different voting rules. those rules may further differ based on which party you're voting for. for example, both the republicans and democrats use caucus in iowa, but the republican caucus rules are simpler so the results are determined rather quickly for them and they all go home. meanwhile, the complex democrat system means its great for prime time viewing as numbers trickle in.
democrats also have "super-delegates" while republicans do not. super-delegates are appointed by the party to vote for what they believe is the party's best interest rather than by the will of the party's members. this was adopted after carter lost to reagan in 1980 as carter was popular with the more hard line wings of the party, but moderates didn't like him (and in general elections, moderates matter more than party hardliners.). this helps the party pick a candidate who appeals both to the hard liners and the general voters, and avoids alienating one or the other. republicans allocate all their delegates based on the proportional winners in each state (which is how the normal delegates in the democratic party are allocated too).
微信公众号红包扫雷2020 has new rules for how super-delegates are used. previously, at convention, super-delegate votes were included as part of the delegation of votes in each state (so if candidate a has 9 delegates and 3 super and b has 3 delegates and 10 super, it's reported that a has 12 delegates and b has 13 meaning b takes the state, despite a being three times more popular). this is the reason many sanders voters felt cheated in 2016: hilary and bernie were rather close by ordinary delegates while hilary was leading by miles with super-delegates.
微信公众号红包扫雷the new rules were implemented to "fix" this (both sides of the 2016 fiasco would agree fix is an appropriate word, but would differ on the definition). the new rules now bar super-delegates from being allocated in the first round. if a clear winner emerges, then the super-delegates are never used. if the convention is brokered, however, super-delegates are in play in round two. in effect, this rule delays the counting of super-delegates in a way that would let a less popularly favored candidate b winning over less party favored candidate a (i'll need to recheck the numbers, but i believe it would have still resulted in clinton winning in 2016. it didn't help that the 2016 convention started on the bombshell that party leadership had been placing their thumbs on the scale for hilary when they should have been impartial).
微信公众号红包扫雷bernie supporters are suggesting this is an attempt to fix the vote for a more moderate candidate because the delegate spread is too wide to win a clear victory, thus going to round two which is suspected to not go in his favor (bernie's independence and anti-party leadership stances will not play well with the delegates that are selected by the party leaders to protect the party from a hard liner who might cost their nomination the general election when a moderate would not.).
just for the sake of comparison: republican brokered conventions typically open up the floor to delegates switching their required vote for candidate to a preferred vote, essentially making all ordinary delegates into super-delegates. though it also has the benefit of allowing delegates who's candidates dropped out to go to more viable candidates. it's a lot more chaotic at this point, but since the super-delegates don't exist to begin with, it's a lot easier to meet the threshold for a clear win. in 2016, the strategy of the last two primary hold outs was to try and deny trump enough delegates required to vote for him to enter the brokered rules and let the winner of that chaos take the nomination. ultimately this did not pan out and trump was nominated on the round one voting.