Hi Linny - you have found my 'secret sit-ups'
I use these as part of a desk exercise session and my colleagues love it - you can feel it and it IS working the rectus abdominus: so no six pack but definitely better posture and strength! You will need to zip up and twist to activate the obliques and transversus - to get that even deeper muscle workout!
Best part is you can do this anywhere - standing, sitting, lying down. You just have to watch out for the 'constipated' look as you concentrate too hard, until you get the hang of it!
Sit/stand tall - lengthen the spine
Pull in the belly button - until it touches the spine
Imagine a zip from your pubic bone up to your belly button
Now slowly zip it up
If you want to make it harder zip it up then twist to one side, return to centre, twist to the other, or do side bends, then unzip - repeat!
All of which is easy to do in an office chair - cos you are just moving papers, picking up something etc etc - no-one will ever know you are doing sit ups yey
And they are definitely effective - and are used regularly in adapted exercise classes - for older, unfit or disabled people!
Have fun, stef