The br option creates simultaneous breathiness whenever voicing is turned on. Another type of breathiness occurs only at the ends of sentences and when going from voiced to voiceless sounds. This type of Lax breathiness (lx) option is controlled by the lx option in %.
A nonbreathy, tense voice would have the lx option set to 0, while a maximally breathy, lax voice would be set to 100. The difference between these two voices is not great, but you can hear it if you listen closely.