Bodybuilding question re: bicep pose

How come when bodybuilders do the "double front bicep pose" (is that what the name of the pose is?), how come they keep their thumb inside their closed fingers, rather than just making a normal fist?
Looks like a hoop grip.

Just curious

The use of the thumb helps "pop" the forearm muscles, which are part of the pose.

 C.S. has Front Double Bicep Posed the correct.



Here is the Blood & Guts Man Himself hitting the exact pose.





The Front Double Biceps Pose


Dorian Yates, 6 times Mr. Olympia performing the Front Double Biceps. Awesome!



TAKU

That guy must eat alot of peanut butter lol