How a fluid bicycle trainer works. Fluid trainer explanation. 

I still shift gears and have been riding my fluid trainer for years. In order to get harder resistance you must shift, which works along side the fly wheel in the fluid.

That is correct.

I have taken one apart and the problem is you cannot refill and properly seal it back up on your own. That's why they send you a new one.

@@ChrisEnockson thanks, I understand that. I am not needing to remove the fluid, so was hoping that there might be some way to fix the unit that did not require draining the fluid or cracking open the fluid holding side. The problem is on the flywheel side: the axel is not secure and it shifts under pressure. It has gouged away a lot of the black plastic and seems as if a bearing was not installed or something... I was hoping to pull the axel out and see but cannot find a way to get the flywheel off, even with the nut removed.

No they adjust automatically based on how hard you pedal.

Not really designed to simulate hills it's just designed to mimic what regular road cycling does.

I guess what I'm wondering is...can you at least stand and peddle for a few seconds without spinning out?

No you do shift!

This video strictly discusses how the fluid trainer provides resistance. High cadence workouts can be done on a fluid trainer and there are fluid options available that offer power.