The bracelets aren’t explicitly explained in the film, but like all Wakandan tech, they are based on the power of vibranium.
Perhaps it’s due to the power of the heart-shaped herb and the vibranium radiation that may indicate the presence of an Infinity Stone.
I’m not sure how the vibranium countered the bullet in Everett Ross’s spine, but there are is definitely more at play with this tech than just a strong metal.
Probably the most impressive use of the Kimoyo beads in the film came when Nakia used a bead to stop Martin Freeman’s Everett Ross from bleeding out due to a gunshot wound in his back.