How does oak barrel work exactly?

It's pretty clear that at max, it has a 10% rate of being spotted by a mob. But how often does the mob "hit" you? Like when you normally get hit, your char blinks and you'll not take hit during the blink. What's the time interval between each hit while in barrel?

Also does ducking down while in the barrel really help(if so, by how much?)? In the description it says "if you're lying down, it's literally impossible to tell the difference", tho I don't see any difference.