Currently if your knock your target prone or daze him, you can shift one square away and he can not attack you in melee in any way.
Daze, yes. Knock prone, no. If the target doesn't mind staying prone (with all attendant penalties), he can crawl one square toward you and then attack from prone.
But if you're behind the big rock in question, you won't have line of sight to utilize these options, so you need to move out to gain that and surprise round is over.
That's why you should be hiding someplace where you have line of sight to the enemy. Of course, that means the enemy has a chance to spot your ambush with passive Perception, but that's the risk you take to get that first round of attacks in. You can have an ambush that can't be detected in advance, but doesn't give you a surprise round; or you can have an ambush that grants a surprise round, but can be detected in advance.
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