"The lesson we’ve drawn from the GHD sit-up is that in spite of the primacy of hip flexors over trunk flexors, or the abs, in this sit-up it recruits the abs powerfully in two ways. First, the movement takes the trunk from hyperextension to full flexion, albeit with negligible load. (No crunch can match this range of motion.) Second, the role of the abs in this sit-up is powerful and largely isometric—i.e., they stabilize the torso from undue extension.

This second point is consistent with our belief that the most powerful, functional, and developmental contractions of the trunk are isometric, not isokinetic. Our favorite ab exercises are predominantly stabilization or isometric exercises. The GHD sit-up, the L sit, and the overhead squat share this stabilization role. The lack of trunk flexion in these moves hides their potency from the uninitiated."

https://brokenscience.org/the-glute-ham-developer-sit-up/