Maran HaShulḥan 'Arukh, citing the Talmud Bavli (Niddah 13B), states that a man must avoid holding his member when urinating (Oraḥ Ḥayim 3:14). That is, holding me'atarah u'lema'alah is forbidden while holding me'atarah u'lemattah is permitted (using the Gemara's terms).
MishnaBeruraYomi.org (2:17-2:57) and MishnaBerura.com (18:30-19:25) both explain me'atarah u'lema'alah (the forbidden area) to be the shaft until where it connects to the glans ('atarah). RaSh"I explains the Gemara accordingly (s.v. מעטרה ולמעלה).
Considering that the glans ('atarah) contains the highest concentration of nerve endings in the male body, how does this make sense? Would it not make more sense to permit holding a place with fewer nerve endings and to avoid holding a place with more nerve endings (to avoid stimulation)?