I think he likely can release more haki than usual, similar to what Zoro does for "King of hell: Three sword dragon" and what I think Kaido was doing for "Destroyer of Death thunder bagua". So that he has two levels of attacks: 1. Basic CoC attacks and 2. greater CoC attacks
In the SBS from chapter 1013 (I think) Oda drew the 4 emperors and Shanks happened to have an extremely big haki trail attached to his sword, which pretty much gave it away already.
And I think similar to KoH and Asura, he and Mihawk likely have some unique abilities aswell, most probably thanks to their swords. And he prolly can use either CoC infused sword slashes or ID CoC haki blasts channeled through his sword I think.