It looks like they are charring the barrel

https://www.angelsenvy.com/guide/whiskey-history/why-charrin...

> People have suggested that the inside of barrels were originally burnt to remove the leftover flavors of goods previously stored within, which sounds reasonable enough ... So why do barrel coopers still char the interior? ... Charring the wood actually primes the wood, which impacts the spirit’s flavor in several important ways that have nothing to do with smokiness ... charring essentially opens the wood up, making it easier for bourbon to extract flavors.