"...if there is no deficit to the hormone". It sounds like if someone has lower-than-normal testosterone then it increases sex drive. But if someone is already at normal levels of testosterone takes more, it doesn't significantly change sex drive.

I assume it means moving up but still starting from within the normal range?

Only one data point, but over the course of this year, I've gone from having the very lowest T level in the acceptable range to the upper half of the range and noticed very little except some increased assertiveness.

Well, getting T back to the normal range restores libido, but fluctuations inside this range don't seem to correlate with libido.