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there are two ways to do a sit-up:
I've been doing them wrong my whole life.
I'm wondering what sort of difference I can expect in my abdomen. Which affects what muscles differently?
You shouldn't do sit-ups at all. Sit-up are mainly done by your hip flexors, but these are strong enough most of the time. Do really target your abs, you should do crunches. https://en.wikipedia.org/wiki/Crunch_(exercise).
Every time you flex your hip (angle between legs and hip), it's done by your hip flexors. Every time you flex your spine, it's done by your abs.
To lessen the strain, I tend to do them laying at a 45° angle. I can keep my back straight and slowly sit up and lay back down using my abs without using momentum. My upper body weighs more than my lower half and I tend to use my legs more when they're held down or propped under something. I might have to do a few extra, but doing them in this position allows me to use my abs more with less arm and leg use when sitting up and less stress/pain on my back.
Fact is, neither of these sit ups realistically will build any muscle in your abdomen unless you are either relatively weak(doing under 20 sit ups), or if you are using a weight plate or kettlebell. However, they still are a form of exercise, so your gut might disappear if you do them consistently, but not because you are building muscles(most likely), but because you are burning a little, tiny bit of fat every time you do a sit up. Sit ups are harmful to your lower back though, anyway, if you want defined abs you should try doing pull ups, barbell squats, deadlifts(only once a week), push ups, and dumbbell/barbell rows. You also should also combine this with cardio and keep a fair diet.
