bracelet如何理解？

FIG. E4 An engineering schematic of the magnetic sphincter augmentation device. The device consists of an expansible bracelet of magnetic beads designed to be placed surgically around the exterior surface of the distal end of the lower esophageal sphincter (LES). Each bead is composed of a titanium case containing a magnetic core of small, disk-shaped magnets. The beads are connected by titanium wires of specific lengths that limit the distance any two individual beads can move apart. When the device is closed (A), the magnetic force is sufficient to prevent effacement and opening of the LES yet is weak enough to allow the device to open (B) with the esophageal peristalsis. When the device is closed, the Roman arch construction prevents compression of the esophageal tissues.

图E4.一种磁化括约肌增强器的工程示意图。该装置包括一个可膨胀的环形磁珠，旨在通过外科手术将其放置在LES远端的外表面。每颗珠子都由一个外面钛壳内含一个由小圆盘状磁铁的磁芯组成。这些珠子由特定长度的钛丝连接，限制了任何两个单独珠子移动的距离。当设备关闭时(A)磁力足以防止LES消失和打开，但又足够弱以允许设备打开(B)随着食道蠕动，罗马拱形结构可防止对食道组织的压迫。

bracelet如何理解？后一个When the device is closed, 是否多余，因为已经并列了？