Lasso anti invisibility

Diana’s eyes were closed.  She stood with her face tilted to the wind, listening, the lasso moving in a lazy, loping rhythm.  The rope seemed to glow in her hands as she released it.  It cut a long, shining path against the gray sky, then dropped around the neck of a huge white horse that hadn’t been therea moment before.  It was as if the lasso had caused the horse to appear
…
“Yes.  When a rider sits one of the phantom herd and takes hold of its mane, she becomes as invisible as the horse.”
“How can we see her now?”
“The lasso.  It always shows the truth.”