Waist trainers ironically de-train the muscles of the ‘waist’ as opposed to ‘train’ it.
They should infact be called ‘waste’ trainers.
Much like if you put a plaster cast on your arm it creates atrophy (muscle wastage) of the musculature around that area. Given that your limbs rely on the the midsection as a point of leverage overall means….
You have less active tissue therefore burn less calories daily (not a significantly huge amount but it all helps)
You have a weakened midsection therefore what your limbs can handle in a free form movement becomes hugely limited (you become weaker overall).
Your surrounding musculature has to word so much harder as synergists and stabilisers hence the chance of injury goes up.
The muscles of the midsection primarily the rectus abdominus and erector spinae (most things we do are in that plane) are both long ‘FLAT’ note the word ‘FLAT’ muscles so unless you’ve got a huge amount of thickened muscle from all those years of strongman and powerlifting the difference it will make is negligible.
If you’ve had results and you want to attribute it to that go ahead. You dieted and trained your ass off and want to pass the credit to an ‘implement’ feel free. I’d say it was totally down to you training and dieting your proverbial ass off.
Waste (sorry waist) trainers are a gimmick however you waste money, waste time and waste muscle