My understanding of the reason why extension may also increase the patients pain with disk problems in the acute phase is that you can get a "pinch" effect where the area which is protruding posterior gets pinched with extension- even though the movement is unloading the disk.

Therefore its often more comfortable for patients to have a pillow under their stomach in prone or with knees bent in supine. Starting out with gentle extension exercises so as not to flare up the condition.

It really is important to note whether the pain is centralising- this represents a good prognosis. If the pain is not centralising but i suspect disk is the issue i will still get patients to perform gentle extension exercises until next session where hopefully i can get a better assessment in as they are not so irritable.