Many people do twisting sit-ups or crunches to target both the rectus abdominus (abs) and the obliques at the same time. Killing two birds with one stone? Makes perfect sense, but there's a problem.
When you do a sit-up – or a full crunch where your lower back doesn't stay flat on the ground – your lumbar spine rounds forward, which is called flexion. The problem is, spinal flexion puts a lot of pressure on the intervertebral discs. But there's one specific motion that's far more dangerous to discs than flexion: flexion combined with rotation. Unfortunately, that's the exact motion you're doing when you do sit-ups with a twist.
Flexion with rotation pushes the nucleus pulposus – the jellylike center – of the disc posterolateral (back and to the side), which is precisely where discs tend to herniate. Unless you actually want a herniated disc – and experience the numbness, tingling, and excruciating pain that goes with it – avoid sit-ups with a twist, or any spinal flexion combined with rotation.