Let's say you have 2 users each with 100,000 HP, Alice and Bob. Bob is voting selfishly, either his own posts or just randomly upvoting posts. Alice judges Bob's behaviour to be harmful enough that it is worth sacrificing some of her own rewards to counter Bob's.

Alice has tried talking to Bob, but didn't make any progress. In fact Bob started downvoting Alice's posts for merely suggesting that he should change his voting behaviour, which he says is censorship. Alice could in the current system downvote everything that Bob upvotes. We have seen how this plays out, it 'works' to a degree, but is pretty shitty for all involved, especially Carol the third party who posts paintings that she makes, who finds herself being upvoted by Bob and downvoted by Alice. As far as Carol is concerned, she is being told her posts are bad and abusive for no reason, and Alice is the bad guy.

Stake-vote-negation is a separate option from downvoting. Instead of having to direct her attention to everything Bob is doing, she applies a 30 day negation of 100,000 HP to Bob. That 100,000 HP is no longer counted for her own votes, but Bob also gets a -100,000 HP on his votes for 30 days. It's effectively timeout for both of them. Bob can still power down, he still gets the basic stake reward, but can't get the curation rewards he expects from his 100,000 HP.

Now to other users, Bob is just a minnow randomly voting on posts. Carol doesn't even notice that he is voting for her at all, nor does she notice Alice, who doesn't have to downvote her.

There are ways for stake-vote-negation itself to be abused. For example, a whale could maliciously negate the stake of random small users for no reason more than to troll the community. However this kind of abuse is itself quite easy to negate by other whales by providing delegations to users who experience this kind of abuse.

Stake vote negation also makes it easier for a bunch of smaller users to act collectively to police abuse. If there are 10 users, each with 20,000 HP, all 10 of them can each negate 10,000 HP of Bob's (100,000 in total). They can then keep posting on Hive like normal, but with only 10,000 HP of their own power for upvotes and curation. There isn't a need for one person to completely sacrifice their life on Hive.