He obeyed the order because he lacked the strength to do otherwise.
Piers slowly slackened his grip.

"Now," he said, speaking between lips that scarcely seemed to move, "you will make me that promise." "What--promise ?" Gaspingly Tudor uttered the question, yet something of the habitual sneer which he always kept for Piers distorted his mouth as he spoke.

He was not an easy man to beat, despite his physical limitations.
Sternly and implacably Piers answered him.

"You will swear--by all you hold sacred--to take no advantage whatever of me while I am away.

You had a special purpose in view when you planned to get me out of the way.