It is rotating 360 degrees per loop putting it back where it was before the screen draws.
rem Rotate The Cylinder
For T=1 to 360
Yrotate Object 1,T
Next T
goes from 1 to 360 each loop so the final rotation of your object will be 360 degrees each loop. (just like it was) Also yrotate spins the cylinder so you can't really tell if it is spinning with no texture try this to see it.
rem Set Up Synchronization
Sync On
Sync Rate 30
rem Make The Cylinder
Make Object Cylinder 1,10
Position Object 1,10,9,20
rem Color The Cylinder
Color Object 1,RGB(255,0,0)
rem Position Camera
Position Camera 13,20,-10
rem Begin Main Loop
T = 1
Do
rem Rotate The Cylinder
Xrotate Object 1,T
inc T, 1
if T > 360 then T = 1
rem Update The Screen
Sync
rem End Main Loop
Loop