I believe Malikis do this however I saw a brother who crossed his hands doing the index finger movement not vertically like most people but left to right to left to right... Malikis pray with hands by their side in Qiyam.
this move like a hummer on the satans head (Devils head)
while you are praying a devil call (Khanzab) stand next to you and start to remember you things you forgot it to take your attentions and destroy your focus
so when you end your Sallat (Pray)
and sit to tashahud this move like a hummer on his head
and when you reach اشهد ان لا اله الا الله و ان محمدا رسول الله you put ur finger up
و الله اعلم