I explained in my first spin class post that each bike is different. For the sake of keeping things simple here, I refer to resistance as being "flat" (very light- enough that you feel a little push under your feet), "light", "medium", "hard" or "heavy." When I put to add a gear, I basically want you to increase your resistance to the point where you feel the change. Those new to indoor cycling may not wish to add so much resistance in their first few classes. If you've been doing this for a while, then you know that you need to add a bit more with each gear change.
Position 1: Seated, hands resting in a comfortable position on the handlebars.
Position 2: Straight up out of the saddle, standing with a slight bend in the hips but mostly upright. Hands resting on the handlebars for balance but not gripping.
Position 3: Hinged forward at the hips, neutral/flat back, hands resting on the far end of the handlebars with a secure but not too-tight grip. Make sure knees don't come forward past the toes (if they do, you're leaning too far forward).
Quad Isolations: Standing in position 2 and trying to pedal without bouncing. It is important to do this with light to medium resistance on your bike. If you feel any knee strain, skip this exercise. This will target the quads and should feel tough (a dull ache or burn is a good sign) but not painful.
Hovers: These target the booty. Starting from position 3 slide your rear back over the saddle and hover just above it (if your butt grazes your seat, that's fine). Rather than pushing your pedals, focus on pulling. You should feel that wonderful burn from your glutes down through your hamstrings.
For the most part, all of my classes include a variety of all these things. The main factors that change from class to class are intensity and duration of each drill. Once you get a handle on how to mix it up on the bike, try putting your own workout together!
As I finish my certifications for personal training, I hope to use my blog to reach out and help others with workouts and gym FAQ's so if you have any thoughts or questions, let me know! Contact me through Google+ or follow me on Twitter.