When. — Literally, in the turning of mine enemies back, which may be either when they turned, or because they turned, or possibly with both ideas combined. The older versions have when. Psalms 9:2-3 form one sentence, “I will be glad and rejoice in thee ... when mine enemies are turned back, (when) they fall and perish at thy presence.”
Fall. — Better, stumble through weakness. So the LXX., “are weak.”