so I'll take a stab... first of all, don't use EPI in closed angle glaucoma, it's meant for open angle (the slow progressive form of eventual blockage of aq. humor outflow). Epinephrine simply increases this outflow, probably by uveoscleral veins (according to Trevor/Katzung) -but it's contraindicated in closed angle
I didn't exactly find direct mechanism for how this works, but from First Aid, Lippincott Pharm Cards and Trevor/Katzung, that's about all I could find