Angle grinder art - GIF

[Frank S]

desbromilow; what you are seeing is an effect known a partial "aliasing". This is caused by harmonic matching of the frame to frame advancement of the video if you could slow the video to single frame advancement you would see stop action movement the reflection of the grinding disc is picking up the writing or other markings on the disk.You really notice "aliasing in old B&W westerns where the spokes of a stage-coach or a wagon always look like they are moving backwards in the movies when they are going really fast in chase scenes trying to escape the outlaws.
As for not having a guard fully half of my grinders do not have guards a lot depends on how I need to use them dictates whether or not they will have a guard mounted on them. Many times a guard is more in the way than the amount of safety it affords to the user.