The 20 minute plank is exactly what it sounds like - 20 minutes of planks. Prepare you abs and core to be ruined. This one is ROUGH.
20 minutes of planks.
Start a stop watch and don't stop it
If you break the plank stop. You can break & reset, but you can't leave until you've held "20 minutes" of total time in a plank position.
If you need to break this up into 40 sets of 30 seconds a piece, then do that - your only goal is 20 minutes of planks total.
If you can actually do 20 minutes straight of planks, you're either a liar or you should be a model on this site. Most people can't do this in one straight go - especially with good form. Your goal should be to do as much as you can, in as good of form as you can, with as few breaks as possible.
It will be hard, but it's not impossible. You will get tired, you will want to quit. The only question is "will you?"?