The Half Curl (aka “chest lift prep”/“ab prep”) is a segmental thoracic flexion in supine. You maintain a stable pelvis/ribs, then curl the head/shoulders until the inferior tips of the scapulae are just light on the mat. Range is modest; quality is everything.
Both drills are prone posterior-chain exercises. One Leg Kick emphasizes hamstring curl + hip extension on a Sphinx/forearm support with thoracic lift. Double Leg...
The Body's Blueprint
Unpacking the Musculoskeletal System