If you put too little voltage across an LED, nothing happens, as I would expect. However, I've seen a demo where a red LED is powered with a 1.5V cell, and it lights. I assumee that red LEDs are 1.9v or so, and was wondering whether there is a range in which they can glow a little bit, and if so, what accounts for it.
I'm betting what you saw was a joule thief.
However, small amounts of current do flow below the forward voltage (see image) of an LED so perhaps it could very dimly light before hitting Vf.
The reason, that you can use diode below saturation voltage (knee voltage) is that the current value depends on voltage by Shockley diode equation. See wiki