I'm really confused.

What little experience that I have had with contemporary upright vacs, if the cleaner is latched in the upright position, the brush roll stops spinning, in order to protect the carpet.  I picked up a used Windtunnel to add to the collection, as it was made in Ohio (I owned a Windtunnel many years ago, but as it didn't care much for the berber carpeting in my old house, I gave the machine to a friend in need who ended up trashing the machine...but that's another story), and found that this machine, while according to the graphics on the label "when handle is up, brush roll is off," with the handle latched in the upright position,  the brush roll continues to spin, except when the machine is on carpeting, and the brush roll stops.  Tilt the machine back, while still in the upright position, and the brushroll begins to spin. This doesn't seem right to me, that the brush roll stops due to the resistance of the brush against the carpet. 

In normal vacuuming, the brush roll works fine, and does not bog down as if the belt is old and stretched beyond its life expectancy.

Any thoughts as to what might be wrong?

Thanks loads,

Joe