I've actually found that one cup of a specific type of coffee, namely instant coffee, drank immediately after breakfast, nearly always guarantees a bowel movement.

I've actually found it to be extremely useful part of my daily routine and when I need to break the routine for whatever reason (I live nomadically so my routines are always changing), my whole day is usually a lot more uncomfortable (i.e., my digestive system and bowls feel slower).

I still haven't identified how all this works, but it's an interesting anecdote.