The surgeon was most likely doing arthroscopic surgery. This means a special tool (an arthroscope) was inserted into the joint to allow the doctor to look inside.

The drive-through sign refers to the ability to pass the arthroscope easily between the humeral head (round ball at the top of the upper arm) and the glenoid cavity. The glenoid cavity is the shallow cup the humeral head fits into. Together the humeral head inside the glenoid cavity forms the shoulder joint.

The location of a positive drive-through is at the bottom of the joint. This is where the lowest part of the glenohumeral ligament is located.

If the arthroscopic probe can move easily through the joint from back to front it's a positive drive through sign. This sign tells the doctor the shoulder is loose or unstable. After treating the shoulder, the doctor looks to see if the drive-through sign is gone. If it is, this is means the shoulder is tight enough.