I think I know what you're talking about. I did this once to get back into a dream I was having after waking up. I wouldn't have thought of it as tensing the eyes, but focusing on the image intensely I did feel some strain. The result was successful, but after many unsuccessful attempts using this tech. from full wakefullness to dreaming, I gave it up. I get to sleep quicker by ditching the eyestrain and loosely holding an image in my head. Doesn't result in lucidity, just sleep.