The decision to perform pulpotomy or other form of treatment like root canal or extraction entirely depends on signs and symptoms. These are often found using radiographs and looking at bone status. Pulpotomy refers to the removal of a diseased portion of the pulp and leaving behind the rest.
If any of these procedures are carried out under proper care and maintenance using anesthesia with the right technique then pain is negligible.
NOTE: Let me add that I am a doctor and although I have some basic idea about Dentistry, I'm not an expert. I consulted a friend of mine who is a Dental Surgeon at JPMC.