Ok. I know almost everyone who read the title is currently typing out a big "no" as an answer. Possibly typing something about how it doesn't help with fat loss and risks reorganizing internal organs like a corset. But hear me out.
I know waist trainers have a bad reputation in recent years, and deservedly so. However, there have been some very prominent coaches in the bodybuilding world that seem to be using them as of late.
Adam Bonilla of Team Elite Physique is a big proponent of using them for his bikini competitors while training. His claim is that it helps them disengage their core which helps keep their waist small while they can grow glutes and shoulders.
IFBB coach and competitor Jonni Shreve uses them for... the exact opposite reason. He claims it helps him engage his core and brace against it. He uses it much like a weightlifting belt, but since they're much lighter weight and thinner they can be used for an entire workout.
This seems to be backed up by elite powerlifter Joe Sullivan who actually sells a product (which I won't mention here) that is essentially a waist trainer. It's a wide cloth belt with velcro straps. He claims you wear a full weightlifting belt for squats and deadlifts and his belt for the rest of the workout to train you on how to engage your core.
Jonni and Joe seem to do it from a more safety perspective because they're both really strong athletes. They're lifting much heavier weight so they may feel the need to have the extra brace. In Jonni's case, if his split requires him to a lot of rows focused on the upper back, he may prefer to use one so his lower back isn't limiting him, thus being able to do more reps without tiring out his back.
On the opposite opinion, Paul Revelia does not like using them at all. He says at best they do nothing at all and at worst the could negatively impact your organs over time.
Of course the opinions of a select few successful coaches is just an appeal to authority. They could just be latching on to whatever they can in hopes it gives them an edge.
Is there any practical use for waist trainers to be worn during a workout? Has their effects been studied?