Essentially, Turtle Breathing is a training method for men to draw qi in through the penis to ren-1 huiyin and to the sacrum.  This practice is absurd, beneficial, and has classical precedent.  :)

Prerequisites

1.  Any version of relaxed breath retention which activates ren-1 huiyin.  Even if the retention of breath is of very short duration, just so long as it effectively activates huiyin.

2.  Sacrum breathing.

The above two should be practiced for at least several weeks before including turtle breathing.  This will help clear stagnation from the lower jiao and activate huiyin and the sacrum.  Then the energy generated from turtle breathing will have somewhere to go.

Once you're ready to include turtle breathing in your routine, do 1 & 2 followed immediately by turtle breathing.

This is a very relaxed technique.  It's done in an unaroused state.  Ideally, the practice should be done outside, in cool air of early morning, with the penis directly exposed to the fresh air.  (Local weather, common courtesy, and indecency laws may oblige you to occasionally stray from the ideal.)

Place your attention at ren-1 huiyin; your breath should be "based" there.  Slowly inhale and - with your mind - draw the chi from the air in through the tip of the penis down along the shaft towards ren-1 huiyin and the sacrum.  No air actually goes in, but the cool chi does.

This takes practice and you'll naturally progress in stages from being able to just draw into the tip at all, then gradually to and past the urogenital diaphragm, then to huiyin, then to the sacrum.

You'll find that as the air-chi passes the testicles that a natural interaction will occur (and cascade up through the vas deferens etc) without the need for any direction on your part.  In fact this activates coolness like a kind of super testicle breathing, but it's not where you should place your focus during the exercise.  Simply draw the chi progressively along the shaft through hui yin and the sacrum.

The Turtle

This solo technique has a number of benefits, on its own.  

        The chi from the air (the "metal" element) has strong alchemy with the energy of the lower jiao (water), with sexual energy in particular.  You'll find that this exercise strongly clears stagnation from around the urogenital diaphragm, the prostrate gland, the testicles, and the lower abdomen in general.  If you have substantial stagnation to begin with, this technique may cause some initial discomfort as the stagnant chi is dislodged.  This is one of the reasons for the pre-requisite of relaxed breath retention: to at least partially clear the way prior to this technique.  If you feel discomfort through this technique, take care to pace yourself compassionately, and to be sensitive to your body's own unique processes and needs.

        The three methods (hui yin, sacrum, turtle breathing) often result in strong unaroused pulsing throughout the sexual area.  It's a good substitute for aroused activity, especially to get you through the morning.

        In addition to circulation, the chi from the air will tonify the water element as well.  It's invigorating.

So.  These three breathing methods together form "the turtle breathing suite":

    hui yin activation through relaxed breath retention, 

    sacrum breathing, and

    turtle breathing

bring cool freshness (from the outside air), refinement, circulation, and tonification to the male sexual region.

The Cool Male  

I made the major revision of this essay in February, 2010, have had this website since 2000, and have been interested in jing refinement methods since the mid-80's.  In all that time the above small set of breathing methods is the most effective for male cooling that I've found.  I think that this is typically men's most challenging (and often over-looked) requisite first issue: being able to cool down in order to more fully & effectively experience the yin arc of their sexual cycle.

Typical male trouble areas, see Healthy Sine essay.

Turtle Breathing has rather obvious application for dual cultivation: the absorption of female sexual energy by the male during intercourse, with the intent of balance, and within the context of mutual exchange.

The other obvious aspect of this is giving energy, emanating masculine heat from the penis.  SimonV's HT-USA post #12205 talks about this.  I leave it to your explorations.

RELATED PRACTICES:

    Activating Ren-1 Hui Yin through relaxed breath retention.  A powerful practice that clears the way so that the other lower breathing practices can actually work.  Yi Swallows Chi is one variation of relaxed breath retention.

    "Vase Breathing", described in Tsongkhapa's Six Yogas of Naropa, includes mixing the qi drawn from the air above with that from below.  Quote, "from below one draws in air gently from the two lower apertures [ie, anus and sexual passage]".

    Lin's anal breathing.  Chia's scrotal compression.  Any breathing method that massages the belly.

    Any of the basic massage techniques of the vas deferens, epididymis, and testicles are good complements.

    Of course, any basic version of lower tan tien breathing.

    Any spine work that especially addresses the distance from the sacrum to the kidneys.

NOTE:  I chose the name of this technique on my own, out of playfulness, without connection to any classic technique that might happen to share the same name.