Luckily, the ratchet effect is a mechanical analogy, and not actually relevant to politics.

Democrats managed to get a lot of rights recognized for LGBTQ people, despite heavy republican opposition. I'm old enough to remember hearing about the first federally recognized gay marriage, and the end of don't ask, don't tell.

Democrats managed to pass a healthcare plan (abeit a shitty one) despite heavy republicans opposition, and internal politicking by other "just barely" Democrats.

Democrats managed to balance the budget, despite heavy republican opposition.

The ratchet effect is a cute analogy, but it's not a real thing that is actually taking place. If you want the party to move further left, then you need to vote in primaries, and support further left candidates by talking about them, calling for them, and advocating for them. If your candidate wins, great, but if they don't, recognize that you need to prevent whatever christofascist shitbag Republicans are running from touching that office.