The vow is a form of Hindu Tapa (or Tapasas), a self-inflicted corporal punishment intended to help bring spiritual enlightenment.
Author Gregory David Roberts places Babas inside in his description of Khareshwari in the novel Shantaram.
Additionally, two online photos of Khareshwari both show men inside, not under a tree.
The Standing Baba have a swing-like device that allows them to rest their arms during the day.
Roberts and others describe the Khareshwari tapa as extremely painful and even permanently disabling.