In many places in Talmud Bavli (e.g. Brachos 30b) Tannaim and Amoraim are described as putting their heads between their knees when praying for something, usually miraculous, which then occurred. This position does not occur in the standardized service, and to my knowledge it isn't the minhag of any community today.
Why did the Tannaim do this? Was it just to minimize distractions or is there another reason?