As against it impossible to reverse side ... on a perfectly smooth surface, such as glass, the sucker does his job.
For against, on a glass substrate, stone (smooth), or wood, it is much less effective.
Indeed if we accidentally touches the mug he holds. But in the case of a flourish where we forget the presence of the mug, it reverses anyway. And its shape does not help, there is a much greater chance of reversing an object of this size as a classic mug.
In short, it remains a good mug, practical, easy to carry. But certainly not the magic tool that is claimed in the videos.
