I thought the "not a super detailed response" explanation by craslovell was excellent

Merge mining allows Bitcoin miners to simultaneously mine Emercoin with their mining equipment. So a 51% attacker now needs to control 51% of much larger hashing power, whereas before they would only need to control 51% of the hashing power that had been dedicated only to Emercoin. And on top of it, an attacker also needs to own a huge amount of emc, due to Proof-of-Stake.

So, the combination of Proof-of-Stake with merged mined PoW should make EMC blockchain very difficult to reverse with the conventional 51% type of blockchain attacks.