Hey Neal! Thanks for reading, and great question. Some instructors and trainers like to cue along those lines — press your lower back into the mat — and from my understanding, it's meant to help you counteract the anterior pelvic tilt that will naturally happen during this supine hip extension movement (i.e. the weight of your legs pulls your pelvis forward and your lower back arches). Pressing the low back down is not my favorite cue, though, because as you mention, it does put you into an exaggerated posterior pelvic tilt, which isn't neutral either. My preference is generally to help people understand what their "neutral" feels like, and learn to engage the TA to help keep it that way when force is applied. TBH this can be a bit of a personal preference, and I'm sure there are trainers out there who passionately feel that the posterior pelvic tilt is preferable! And I do think intentionally exploring the full range of motion of the pelvis, in a safe way, can be beneficial — e.g. pelvic tilt exercise. But I like neutral for abdominal moves like the one here :-)