Self-mutilation and other practices such as Kusali Tsog or Chöd where the body is offered may also be about mutilation of 'the self' and the elimination of attachment.
Rather than assume we know their motivation, however, I would be interested to know if there are any teachings by past masters (who have deliberately harmed the body) which explain their actions. I wonder how it may relate to extreme asceticism, and whether any past master has claimed attainments as a result of it.
No there is a big difference between self mutilation and Chod practice. In Chod practice everything is offered including ones own body to demons and spirits but this is only in visualization and one is never encouraged to actually do it. Obviously (although it is said one on the bhumi's can offer body parts there have been very few cases in history if any). It is never a good idea to harm your body or cut of the body parts as an offering to buddha's or anything like that. In fact one would be filled with unbearable pain and would later regret the actions.. It seems that the Chinese cannon had some questionable Sutras that encouraged the practice of self mutilation or self immolation but I believe these were from questionable Sutras that appeared only in the Chinese Buddhist Cannon and not from India. Anotherwords the Buddha taught the respect of the body as a vehicle of awakening. Especially for practicioners of Tantrayana to harm the body is a root downfall, a samaya breakage.
Apologies, clumsy wording. Yes, our physical body should be maintained, without attachment, as a useful means to gain access to Dharma.
I was grouping the actions and practices together in possible intent only, but failed to explain Kusali Tsog or Chod properly, both of which I practice. Without elaborating, once one's 'reality' is that of the HYT Yidam, in Kusali Tsog for example, I would say the cutting away is very much 'real' to that extent. If not regarded as real, why bother, as the purpose is to affect the mind through such powerful actions?
Chopping up the body I deludely perceive as 'real' and 'mine' at this moment (as I use it it type to you) would not, I feel, be more meritorious, and is relatively wasteful, as one may perform the action only once, whereas one may perform Kusali Tsog or Chod many times.
I'm interested that you think self mutilation or self immolation were not recorded in any Indian Buddhist sutras. You may well be right in terms of scripture, I haven't read widely enough to know. I know both practices are widespread in India and have a long history. In the OP, Zen is mentioned as the context, presumably incorporating Chan, which would support the origin of the practice mentioned in the Chinese sources.
I'm not a history scholar, but I wonder if there are other cultural or religious influences at play here, which do not have a Buddhist origin and may have 'crossed over' in the Chinese and Japanese contexts.