There's a category of core strength called "hip flexion with neutral spine." And it's overlooked by most athletes. This category consists of any exercises where you're actively bracing your core/lumbar spine while bringing your knees to your chest. Here's one of the best exercises to train it.

Band Resisted Jackknife

This is probably the most challenging subset of core exercises. The focus here should be on keeping your lumbar spine in a neutral alignment throughout, with no lumbar flexion or "tucking." Proper cueing here is critical. Focus on the following:

  1. Chest up and out. No hunching over to brace.
  2. Lock down your midsection. Allow as little movement here as possible.
  3. Use your hip flexors to "pull" your knees to your chest. Exercises like this can help improve your setup/tension in the hole when squatting and pulling. Maintain neutral and "pull" your knees to your chest.

It's not going to be perfect early on, but really focus on staying as stable and neutral as possible.

Related:  21st Century Core Training

Related:  4 Minutes to Core Strength