Is turning hands upside down when telling bad things in du'a that we don't want to happen is bid'at or it is sunnah?
For example in du'a Salâten Tuncina when they say "afat" which means disaster...
There's no Sahih hadith telling us that the Prophet turned his hands upside down when he told bad things.
Some have actually claimed the Prophet did this when asking for rain, but there's no evidence to confirm it.
So to do it isn't the sunnah and would be considered bid'ah as it doesn't agree with the method of Rasulullah on how to make du'a.
If the act is done thinking that somehow Allah and His Messenger legislated it and it is a part of the deen , although it is not, and they believe that there is an unseen benefit to it, then it would be considered bid`ah.
“Prophet Muhammad (PBUH) was turning his palm up by the time he was asking Allah anything to be done. However, He was turning his palm down when he wanted to avoid something.” (For Hadiths regarding the issue, see: Musnad, Ahmad b. Hanbal 4/56; Majmau’z- zawaid, 10/168; Jamu’l-fawaid, 2/618; al-Fathu’l-kabir, 2/357; )