Extensor action is the process in which the right arm extends/straightens in the golf swing. Even is you have the right arm straight at address, basically once the right arm starts to bend, extensor action is the process of that arm fully straightening until just after impact. So, what's the process entail?
For most golfers the right arm is bending at P3, then folds at a 90* angle at P4, then steadily straightens until just after P7.
Meanwhile, the left arm is straight in the downswing and just stays straight, but 'stretches.' Anyway, here's the video.