Partial ranges of motion can improve activation and recruitment of the core muscles. And it can re-educate your core so that its strength can be transferred back into the compound lifts.
Here's what to do: Use a pad to elevate into a more passively flexed position at the thoracic spine and rib cage. This position will improve the strength curve of the rectus abdominis, allowing a more targeted way to overload these tissues with load or metabolic stress without jeopardizing the spinal position.
Plate Push, Partial Range
If you struggle to engage the posterior chain to stabilize the pelvis, a great variation for the partial range of motion crunch is placing your legs on a weight bench in a 90-90 position.
From this position, add a pad to the back to create the perfect position to gain muscle tension and recruitment, and to safely train the spine using a controllable and strategic flexion. It's best to get the hips flexed to over 45 degrees which helps the pelvis to stabilize more effectively.