5 elements of form

 The slight lean is

The slight lean is arguable.



They are saying to bend from the ankles. This is recommended by some(Jason R. if i'm not mistaken), but others have found a vertical body with a push out from the hips helps also. This is due to the lack of gravity you have to fight.
 
Chi also recommends a

Chi also recommends a slight lean from the ankles. It's believed that if you bend, therefore lean, from the waist, you are interferring with your body's chi/energy due to the bend at the waist blocking its flow.
 
kean: Yes it does. If you

kean: Yes it does. If you bend at the waist your butt will move backwards to compensate and keep you balanced when you really want to be unbalanced and fall forward.

Personally, I think the importance of a forward lean is greatly exaggerated. There is no such thing as free energy, no matter what Dr. Romanov says and your forward momentum is, or should be, created at the hips, not by falling forward.
 
I agree with Jason.  The lean

I agree with Jason. The lean is good for speed, and I find I can't go above a certain pace without it. But a lean while you're running slow just makes you look like you work for the Dept. of Silly Walks.
 
 If you look at a lot of

If you look at a lot of elite distance runners (who are running very fast compared to most people) most do tend to have a slight lean. So I assume there is some value to it.

But I think it is the wrong thing to focus on while learning. I learned the lean idea from chi running (before I did much BFR) and it was somewhat confusing... I ended up leaning from my waist. But since going BF I think I have gotten more efficiency out of really focusing on staying erect and keeping my cadence high. Staying straight up also seems a bit more conservative on rough trails.

Whatever the lean is, it's SLIGHT. If you look at Danny Dreyer, the chi running guy, on youtube, he has a super straight spine, but only a slight lean.

The one place I really consciously try to lean now is while going down steep hills. I doubt I ever actually get leaning truly forward, it's just a mental thing to try to prevent leaning back and braking.