There's no evidence that it works. The handful of studies done are generally small in size, fairly weak and with varying patient populations (sometimes healthy adults, sometimes adults immediately after giving blood, etc), but they've all consistently failed to show (and, by "show," I mean like they might get one subject to have an increase in BP, without evidence of an increase in cardiac output, otherwise no real change, little less a statistically significant change) any increase in blood pressure between laying supine and being in trendelenburg. They have shown, though, increases in intercranial pressure, and an increase in work needed to breath (shifting abdominal contents), among other adverse effects.
Another problem is that different studies test different things. There's a difference between trendelenburg (defined as a full body, head down tilt. Essentially unheard of in prehospital care), and passive leg raising (just the raising of the legs; commonly called "trendelenburg"), but again, regardless of what was tested no difference was found.