Sorry for the delay in my response! We were less active during the holiday break, and we're just getting our gears turning again this week.
"Music tension" is for the dynamic soundtrack, so it can ramp up in intensity when there's more activity in the simulation. Not every track has this element, though, and it may be more noticeable during certain points in tracks than at other points.
A good track to use when testing manual tension is "Trackone" (we'll have better names in the future
). If the tension is at 0, the track is pretty calming, but if you crank it to 1, you'll hear the percussion really kick in, and it generally feels a lot more epic.