Keep arms reaching and lie flat? That would be like saying “If you fall you’d better fall on your face!”. Contradictory.
Mentally follow this: keep your arms reaching the sky. Don’t drop your arms. Now, kneel down while keeping your arms reaching the sky. Now, drop on your belly while keeping your arms extended towards the sky. Now, don’t move a muscle and we will handcuff you.
It is clearer and more simple. It is way better than “don’t drop your hands or I’ll shoot you” and “crawl towards me (thus dropping your arms” so bang, bang.
arms reaching means arms extended way above your head.