Although hitting the knuckle def hurts and all, I'm less scared of that than I am of overshooting the landing slope. I've seen a couple of videos of guys that completely overshot their landings. That actually distinctly resembles a recurring nightmare that I've had all my life. So, it's kind of a good news/bad news thing with me.
I have a similar nightmare, every winter for the last 19 years. I'm riding some epic park and hit a jump 3x too fast and watch the landing go by as I'm still going up.
Try not to tense up and remember to breathe. Just before impact, exhale...this tends to force you to loosen up a little and allow your body to absorb the impact a little easier. When you are on a big feature though, nothing will completely save you.
This is actually a good technique for any hard landing/crash. Exhaling upon impact (for me anyways) keeps me from knocking the wind out of myself, which to me hurts more than a sprain, bump or bruise. I absolutely hate that feeling. Anytime I'm going to hit hard I'll let out a loud grunt, like a GRRRR! upon impact and it's worked so far.
Although it's not a true knuckle landing between both feet, if you feel like you are going to come up just a little short you can kinda bone or push out your front leg and get the board a little more further forward. Word to the wise though, DO NOT land in the full melancholy position with your front leg tweaked like that or you're in for some serious pain, this is only for when you're about to case your back foot on the knuckle. It's difficult to explain but once you do it once you'll understand.
From my experience, taking a knee to the chin is when you kinda just give up and let the crash happen. I'd say it's safe to assume most of us aren't hitting these 100 foot super-kickers we see on TV. With that knee to chin impact it's basically just collapsing into the knuckle upon impact. Remember your legs are strong, and like when riding sloppy stuff at higher speeds you can use your legs as shock absorbers. Or think of it like jumping off a ledge, you're not going to just hit and let your body fold onto itself, so don't do it on a park jump. Don't close your eyes, brace yourself for the impact and watch it come so you can prepare yourself accordingly, hearing (and feeling) the "clack" of your upper and lower teeth smacking together is not a good feeling.
Ok last thing, and maybe I'm overthinking it because of a night of too many PBRs, but thought I'd add this in. For you newer riders hitting progression park stuff. Not only pay attention to the approach, starting point, and speed of other riders hitting a jump, pay attention to the rider themselves. If you see a younger kid pointing it heading towards a jump, it doesn't mean it will work for you too. A smaller kid will need more speed making it to the landing than an adult. Around here progression park stuff is a shorter landing and you'll need to judge your speed accordingly. Usually hitting a knuckle on that stuff won't hurt nearly as bad as a larger park jump so watch your speed going into it, don't be self-conscious about coming up short on your first hit because those of us on the chairlift have always seen much worse, lol. Same goes for different weather conditions. If it's dumping and you need to point it to get the the landing ok, but don't do the same on hardpack. Seems like a moot point to make but honestly have seen people do it around here and overshoot horribly.