They're pretty pointless IMO. Even if they're performed correctly its just hard to argue for ever doing the exercise because there's just so many others that are better by nearly any definition you want to use.
If you do want to do them, be sure not to plant or weigh down your feet at at all. Doing so encourages you to use your legs to lift yourself up rather than fully engaging your core.
Honest though: I'd skip the situps and just do planks. If you want more movement there's also leg raises and oblique twists.
They're pretty pointless IMO. Even if they're performed correctly its just hard to argue for ever doing the exercise because there's just so many others that are better by nearly any definition you want to use. If you do want to do them, be sure not to plant or weigh down your feet at at all. Doing so encourages you to use your legs to lift yourself up rather than fully engaging your core.
Honest though: I'd skip the situps and just do planks. If you want more movement there's also leg raises and oblique twists.