It is called reflective armband, or reflective slap wrap, reflective
bracelet, magic ruler, slap armband ruler.

Use:
Use it when walking, jogging, cycling.

Functions:
1) Hold the reflecitve surface upward and hold tightly one end of
the reflective band.
2) Tap the wrist / ankle gently to let it wind around.
3) To straighten unwind the strap and stretch it until it makes a
sound like a 'crack'.

Warning:
1) Do not place close to fire.
2) Store in a safe place out of the reach of children.
3) Do not use it if the outer plastic peels off and the inner steel is
exposed.
4) Do not put into mouth or use it to poke anyone.
5) Do not scrape off the reflective surface.

More features:
1) The nice reflective items for safety or promotion
2) Easy to use
3) Put your logo on it
4) Width: 3cm (normal)
5) Length: 30cm, 32cm, 35cm, 38cm