Either,
1. He ceased not to fight with that hand. Or,
2. He kept his hand and spear in the same posture, both stretched out and lifted up, as a sign both to encourage them, and to direct them to go on in the work. See Poole on "Joshua 8:18".
Related Commentaries of Joshua 8:26
Joshua 8:26
26 For Joshua drew not his hand back, wherewith he stretched out the spear, until he had utterly destroyed all the inhabitants of Ai.