Abs should be trained by crushing them together (Figure B). I see
folks keeping their lower backs straight when sitting up which
doesn’t really accomplish what they want (Figure A).

Consider, by keeping the back straight, more work is done in the
lower Erector Spinae muscles in the back, than in the abs. Properly
done, the crunch or sit up bends the lower back forward jamming
the abs into one another.

In figure A the muscles doing the “lifting” are in the lower back and pressure builds in the directions of the arrows. Figure B shows the proper way to train the abs. Forcing the upper abs down to meet the lower abs will properly “crunch” them together without affecting the lower back.