A warning that I have not seen mentioned here and which I have come across in China from martial artists, TCM doctors, and qigong practitioners is that regularly placing the mind upon the lower dantian (意守下丹田 in Chinese), especially in young men, can lead to excessive nocturnal emissions and premature ejaculation (I do not know what, if any, problems it could cause for women of any age). Various personal experiences from my own practice in my 20s as well as 30s lead me to put credence in this warning.   There are instructions unique to different schools which can mitigate this problem and take advantage of its causes to transform what has built up and is destined to "spill" for other uses. There are also Chinese schools that teach to avoid placing the mind upon the lower dantian at all. As always, best probably necessary to find an instructor in person if possible (the perennial advice). I cannot offer further details, I apologize.   To be clear, I don't know anything about breathing into the lower dantian, that's not what I'm talking about. But since whatever that is sounds like it involves keeping mental attention there, figured I should share this information.