This is the vault region of the storm. It's an area of weak radar returns nestled between the supercells updraft, core region and forward flank inflow. It's a dangerous area to reside in as a chase, not because a tornado will suddenly form or anything, but rather because this is where some of the largest hail likes to reside.
The larger the hail, the more translucent it will appear and an area of clearing might actually be filled with baseballs or larger masses.
You can actually see striations in the cloud and massive hail stones create hail streaks as they fall through the cloud.