"Overhead extensions" refers to exercises or movements where you bring a weight or resistance object above your head and extend your arms fully. Full definition
And like the dumbbell overhead extension, it also puts an emphasis on the long head. (kingofthegym.com)
In other words, when you do overhead extensions and stretch the long head, you make it much stronger, enabling it to take most of the load when doing the exercises. (fitnessandpower.com)
The long head is fully stretched only when your arm is raised overhead, so exercises like overhead extensions give you an opportunity to really target it. (fitnessandpower.com)