Down or not?

What is the rule these days about how you are ruled down, and the "ground cannot cause a fumble" myth. I cant remember them all, but I can think of many instances in the past few seasons where a guy catches the ball, maintains possession, hits the ground and the ball pops out. The refs rule it a fumble and the other team runs it the other way.

I've heard also that "an elbow equals a knee". Is this also still true? In regards to the Davis play, the ref said he "barely got two feet down before he got hit" then the ball was dislodged when he hit the ground. If he barely got two feet down, then he got two feet down right? The ball popped out when his elbow hit the ground, not when the ball hit the ground, so he's down right? He was not able to make a "football move", but if that play happened on the sideline and the ball goes out of bounds, do they still call it incomplete?

I'd just like to know from someone more knowledgeable of the rules than me, are the statements "the ground cannot cause a fumble" and "an elbow equals a knee" still accurate in the NFL? Thanks

This is a FanPost and does not necessarily reflect the views of Niners Nation's writers or editors. It does reflect the views of this particular fan though, which is as important as the views of Niners Nation's writers or editors.