Trance-like syndrome in a dog can be a spooky sight to behold, but it doesn’t appear to have more ominous effects.
Dogs that exhibit this behavior — called trancing, for short — will creep, as if in slow motion, under materials that skim their back, like hanging leaves, tablecloths or other fabrics.
Trancing is by far most common in bull terriers, but greyhounds and some other breeds can engage in it too.
No one knows what causes trancing. Dogs remain aware during the episodes and most can easily be called out of them, so it’s not like some type of seizure. Beyond that, none of the brief studies or surveys conducted have shown a correlation with other neurologic behavior.
In short, affected dogs don’t appear distressed by trancing, so you shouldn’t be either. It may just be a mystery that stays that way.