Ah, sorry about that. Thank for you for the feedback
Yeah, the effect rewind and fast forward is not that evident especially for the normal slash attack; rewind, for example, has a clearer effect if you use it after the monster's charge attack. Also they don't work especially if there’s nothing to rewind and fast forward.
One way to see if it works is when you see the hero’s health is 0 (red hp bar) and bleeding out, the background will turn reddish and you can pause with space and drag a line from the rewind button to the hero’s sprite. That would bring him back to his previous health. But sometimes, the monster is too fast and the rewind will not be enough to save the hero. Upgrading the pause ability or lowering rewind cooldown, ideally both, can make it more manageable.
If you give the game another shot and it's still unclear what to do, please tell me. I would like to learn what I did wrong and learn from that.