"Prepare to die!"
Start to be ready = start to prepare. Start to prepare = -rupchoH. But the sentence isn't Start to prepare to die; it's Prepare to die. Prepare is just -rup in the imperative. It's telling the listener, "Cause yourself to be in a state of preparedness to die."
Imagine a Klingon told a joke you didn't find funny. He tells you yIHagh! Laugh! With or without a -choH on it, the listener is expected to go from a state of not laughing to a state of laughing. This is what imperatives are all about: commanding that a state or action that is not in effect be put into effect, or commanding that a state or action that is already in effect continue or be discontinued. You don't need to use -choH just because it's an imperative.
yIHagh! would be used to get a single "Hahaha!" out of someone; yIHaghchoH! would be used to make someone start laughing and continue laughing. Likewise, yIHeghrup! would be used to get someone into a state of readiness to die immediately (maybe the death blow is about to land), while yIHeghrupchoH would be used to get someone into a state of readiness to die that then continues. In English, Prepare to die! is used for the former, while Be prepared to die! is used for the latter.