I just had my first spinning class and hadn't realised it was going to be a fixed gear bike, when the instructor was asking us to get out of the saddle (or half out) the necessary technique felt very unnatural and much more 'bouncy' than what I would do climbing a hill on a road bike. Does that mean bad things about my real world form or are they just very different things? And if so will a lot of spinning change how I ride?
It impacts your real world form, but it doesn't necessarily do bad things.
The biggest change is going to be out-of-the-saddle riding. As was mentioned, there is a tendency to bounce up and down on a spin bike, and this naturally occurs when we leave the saddle by dropping our hips down and forward or move off the saddle with our chests vertical. Even in these positions you could stop the bouncing by focusing on spinning your feet while keeping your hips in place, but it's going to look goofy and you're going to waste yourself - like Tony Little on a Gazelle. The better spin bike posture is to raise your butt off the saddle and move your hips back not forward, and keep your back straight and flat (parallel to the ground). Now as you spin your feet you will have a better time immobilizing your hips in space because some of your weight is forward on your arms and you don't get wasted controlling the bounce. Also, to the extent that your hips are moving up and down, because your torso is not stacked on top of them, but rather leaning forward on a different plane, the up down movement is not transmitted to your upper body but stops at your hips. If you tried to use your hips to stop the motion while your torso was vertical your hips would be moving radically side to side like a
salsa hula dancer.
The exercise bike feels different, because it is fixed in place and can not react to your movements like a real bike.
When you jump out of saddle, your body moves forward in relation to bike. On road, the you keep moving at roughly same speed and the bike slows down momentarily to accommodate the change in relative positions. A fixed exercise bike can't move, so you are moving and stopping instead and that is what you feel as jerking motion. Also, when standing on a real bike you rock the bike to keep your body in place, but since this exercise bike does not allow sideways motion you have to move your weight sideways to put force on pedals.
I spend 2 mornings a week on an indoor trainer.It has the ability to change the level of effort up or down.When pedalling off the seat it is important to increase the load or level of effort then back on the seat and using the down button drop the level of effort. To get used to it get off the saddle for say 10 spins then back down for 10 etc. With the higher effort you will be pedalling slower with more tension in the leg muscle groups and you will be gaining better balance skills.