If you do have to intervene, make sure your bator is in a room - door closed and a space heater on to make that room as hot as you can get it.
You need two people to do it with good speed.
bowl of pretty warm water (it will cool before you get going so take two bowls if you're not sure on temp) a syringe will help - you can use it to drop little drops of water directly onto the membrane - be careful to avoid the chick's beak and face. Papertowels.
You fold a few paper towels (so they're LONG, I fold twice in one direction so they're skinny but long), stack em up to keep handy and dip in the warm/hot water and lay it in your hand, fast as you can, have someone open the bator and you snatch the egg out and into the warm wet papertowel. You can wrap the papertowel around the end you're not working on, or just kinda flip it over to keep that egg hot and moist.
Use tweezers to pick of little bits of shell, be carefull not to go through the membrane - just pick off shell first.
Don't keep that egg out for longer than a couple of minutes.
Second person dip another papertowel in the warm/hot water and wrap around the egg - not over the open part. And fast as a flash put it back in the bator.
Wait .
If you need to go in again, you might need to start picking/pulling off pieces of the membrane - at this point if you see ANY blood at all - put that egg back in a new papertowel and back in the bator.
You might need to do the process 3 or 4 times, if the chick doesn't help itself.
The split seconds that bator is open, I've only notices a slight drop in humidity - I don't think I ever dropped the temp at all.
REMEMBER - room HOT with a space heater. enough so that YOU are uncomfortable.
GOOD LUCK