Some time ago I talked to a beginning practitioner of Taoist Neidan. Beginning in the sense that he hasn't achieved much, yet he is a veteran practitioner, because he has been reading, taking courses and practicing for over five years! He was still doing the microcosmic circulation practice, and he didn't know his stage of progress, and he hesitated to move forward, afraid of any side-effect from the state of 馬陰藏相: successful sublimation of one's sexual impulse. Like some practitioners, he wanted to move forward, but he didn't want to lose his sexual potency!
The literal meaning of 馬陰藏相 is a contraction of a male-genital till it is retracted inside his abdomen. To make matter worse, there is a Chinese saying that when such condition is pathological, the person will die!
What actually is the "imagery of sublimed sexual impulse" or 馬陰藏相?
It is the definitive signal of having accomplished the objective of "building the base" (筑基) using microcosmic circulation. Some classic texts mentioned another signal of accomplishing the objective: Seeing bright light twice (陽光二現). The problem with this signal is that it is difficult to judge what is a significant bright light. One will see light simply when one rubs one's eyes! A more concrete signal is 馬陰藏相, the imagery of sublimed sexual impulse.
What is this signal and how can it be felt?
To understand this, we have to get back to the classical Taoist method of doing microcosmic circulation. When one does chi mediation focusing on one's lower abdomen (Dantian) and perineum, in time, one's pelvic floor muscles will be loosened or activated. With enough activation, one's genital will be stimulated (erection may appear). The Taoist way is to focus on chi (that focus will block a more natural mental drift towards erotic thoughts), and direct the chi thus generated to move up one's spinal cord. With more such practice, the chi-channel along the spinal cord will be opened more, allowing chi to flow more smoothly.
With more practice and clearing of the spinal cord chi-channels, chi generated through stimulating one's pelvic floor area will quickly move along the spinal-cord route rather than the natural route of stimulating the genital, and therefore the genital will not respond and no erotic thought will be aroused. This is a successful sublimed sexual impulse (馬陰藏相)! And it won't affect one's sexual potency.