Sitting straight up, not touching bar?

[kymberann]

more weight over the pedals and less over the bars works the core more effectively.
Actually it is more weight over the saddle not the pedals! And this weight comes from the hips and butt area! THink of it as a gentle stretch and the more the glutes are back the more the hamstrings are engaged which helps create balance between quads and hammies. This is virtually impossible to do in an upright position. I very rarely use an up right position as it is awkward and just not standard unless it is in a recovery mode for you back!
Watch road riders, you wont see many in an upright, it slows down timw and impossible to climb hills!

[RoadRaven]

I can't sit up straight or hold the bars on the spin bike for long.

The way I ride the spin bike for long periods of time is spending most of it resiting on my forearms in a pseudo aero/TT position.

The spin bike is so radically different from the road bikes - even though we have tried to set it as best we can to match, it still feels wrong...

[indysteel]

After my own frustrations with certain spin instructors at my gym, I tend more and more to treat their instructions as suggestions. If they don't make much sense or stand to injure some part of me, I freely ignore them. Like the instructor who wants our class to spin like crazy people while using a high resistance. Unless he's paying for my knee replacement surgery, I'm not doing it.

It seems like you've already gotten an answer to the no hands spinning question, but I'd further add that sitting up like really just works your quads. Strong quads are great, but if you want to be really powerful on the bike, you have to work your glutes and hamstrings. Your question brings to mind a passage in one of my favorite cycling/exercise books--"Bike for Life." In the book, it states that if your bike fits correctly, you should be able to practice "butt-centric" cycling. The ideal bike fit, according to the book, puts your upper body in approximately a forty-degree angle to your bars to best utilize your glutes and hamstrings. It makes sense to me, then, to approximate that angle in spin class so that I work those muscle groups. In fact, since I regularly started spinning, I do feel more powerful in that sense on the bike. I notice it during seated climbs in particular.