Bit of a gimmick
I ordered two of these they took a week or so to arrive. The light turns on/off by twisting the top which is where the batteries are housed. the top has a clip used to attach to your dog's collar. I found once the lead is also clipped to the collar the light is at the back of the dog's neck and therefore not as visible to traffic as I had hoped. Sadly only one of mine worked properly the other seemed faulty and only worked sporadically but not really worth the hassle to return. They do come with spare batteries. I don't use them much now and have invested in high vis gear instead. Much safer!! Ok though if you just want to see where your dog is in the dark