From the looks of it, it relys on something pushing rearward on the blades that causes them to pivot out due to that rounded part rubbing against the base of it. Which makes me wonder if they'll lock open (making them illegal most places as a barbed head) or, once the initial push from entering the hide is gone, they'll slide closed making the wound channel basically the 5/8" size of the leading portion. I'd be interested in seeing them tested.
In flight the blades are held closed and secure without any o-ring or clumsy clip assembly. The simple "spring washer" keeps the blades closed until impact and ratchet the blades open, keeping the blades from closing prematurely during penetration through flesh.
I don't know if I trust that or not.