Some lean some don’t.  Beginners tend to sit very upright.  Provide for a good airflow by keeping their legs apart and generally look uncomfortable in the air.  Some more experienced pilots fly like this, and since they are upright don’t put much pressure on their back support.  Why would a more experienced pilot fly like that?

May I suggest the following...

Sit upright on the edge of a table.  Now get two people to take hold of the table on each end and give it a bloody good shaking.... Dasklip in summer!  Do not put your hands down to steady yourself.  Hold your ear lobes;>)  Notice your legs reaction:  Thrashing all over the place to keep you balanced, and as wide as possible to increase your roll stability.

Now, after you have fallen off a few times, lie back on the table.  Resume the turbulence.  Sure you roll a bit left or right, and your reaction is now to straighten your legs, but what an enormous difference.

That is the ideal flying position! (But not practical before you blast me).  Ignore the benefits of reduced drag and let's look at how a pilot will experience the air:  

Turbulence will now just rotate your body left or right, instead of falling from side to side in the harness and having to grab the risers to steady yourself.  To weight shift, you roll your hips - no need to lean your whole upper body over.  In the leaning scenario; imagine having a tuck on that side.  Not pretty.

Many people battle with falling about in the harness.  Instinctively grabbing the risers.  I know that for my first 50 flights I used to often hold onto the risers to steady myself.  It seemed reasonable that if I could steady my body by grabbing the harness with my toes I could achieve the same effect.  Since that would elicit some strange looks, there has to be an alternative.  Yes, a full body cast.... just kidding.

When I first started to use the speedbar I noticed that through minor turbulence, the glider seemed more solid when I used a bit of bar.  Which did not make much sense, until I realised that I was more solid in the harness.  With my feet together, pressed back into the harness, by the speedbar pressure, I did not fall about, and since it was more difficult to yaw my body with my feet in front of me, the glider tracked straighter.  Clue...  Get a stirrup you idiot. Which I did!

Ha! He says:  Lie back... bloody hell!  I can’t see a flipping thing except my wing over my head and am doing sit-ups every time I do try to see past my feet.  Ok, this is not practical, and most of us dont want to buy a supine harness.  The good news is that there is a compromise position.  The modern harness will let you go a bit more supine.  Most important is the stirrup.  This stabilises your body and pushes you into the harness.  You will now be flying with straight legs, and will look like an idiot if you are used to flying with a bucket seat.  Now your legs will be pointing at the sky. The solution is to tighten the straps under your bum.

This lifts your bum, drops the angle of the legs, and makes the back a bit more upright. Ideally the legs should point below the horizontal.

The back now has an angle of about 20 to thirty degrees off the vertical.  Very comfortable, good in turbulence, but the stirrup pushes you into the harness. The shoulders soon become tired if the shoulder straps are tight, so loosen them a bit.  A single strap must now support most of the back, and depending on the harness this strap may be too low causing you to arch your back and strain the stomach muscles.  A possible cure is to route this strap so that it runs around the outside of the back protection (Foam/Kevlar or whatever).  The other is to use a form of protection that does not bend like standard soft foam, and that is a Kevlar plate or the Charly scooter.  The Kevlar plate is considered unsafe and may exacerbate back injuries according to recent studies.

A possible criticism of the straight legged more supine position is that it is easier to spin the glider.  This is not correct, as it is less likely to spin.  It is more difficult for the glider to start your body moving left or right and as a result it tends to be more yaw stable.  The problem is if the glider actually does spin, (so how often do you spin your glider???), your body can't catch up as quickly, and you may get twisted risers.  The immediate  "Oh my God!"  reaction is to sit upright and bend your legs anyway in a situation like this, so it is a moot point.

Craig