What is meant by "tooth flossing" (falaj al-asnan), and is straightening crooked teeth considered "flossing" (falaj)? Is it forbidden?

The prohibited practice of teeth-gapping (al-falj) is the act of separating teeth by filing and abrading them for the purpose of beauty, based on the Prophet's (peace and blessings be upon him) saying: "and those who create spaces between their teeth for beauty, changing the creation of Allah." However, treating dental defects and crookedness with orthodontics or other methods does not fall under this prohibition.