Gotcha!

A deafening sound echoed as the head of the snake hit Eva's arms squarely. 

It was strange that the snake did not open its mouth to attack them but Ban already knew from the start that it wouldn't happen. 

One of the snake's weaknesses was its inability to open its mouth while attacking. The Shiny Headed Rattlesnake dislikes sand entering its mouth, it only opens it when eating and it would even make sure that there was no longer any sand left on the food it's going to eat. 

In truth, the reason it's head became shiny was actually also due to its dislike to sands entering the gap between its snake while burrowing in the sands, searching for prey. With the shiny head, it would already be able to repel almost all sands in its path smoothly. 

That's the reason why Ban was confident about not getting eaten while they were trying to defend themselves.