From Valve Developer Community
Before the autoplay sequences are run, the specified $ikchain is locked in place wherever it happens to be. This is useful for locking the feet in place in case there are any pelvis motions on autoplay head turns or chest turns.
$ikautoplaylock <chain name> <pos lock> <angle lock>
$ikautoplaylock rfoot 1.0 0.1 $ikautoplaylock lfoot 1.0 0.1 $animation a_pose_neutral "CT_body_poses" frames 0 0