In this essay I survey major areas of exchange, provide corresponding techniques, and prompt you to explore.

One of the facts about working with alchemical opposites, is that they get stimulated towards each other when they are proximate, but when they actually mix they unify into something stable (the "elixir").  This leads to important insights about exchanging sexual energy.

When you're stimulated by your partner's energy, their energy has come into your perceptual field.  The natural predominant response, especially for a man, is for your attention (and sexual energy) to go out to seek mixing.  If a man just follows that impulse, he'll only be on a cycle of building, exciting, and expelling male sexual energy, never really attaining the nurturance, power, stability, and love that comes from deeply receiving feminine energy.

One of the keys to successful exchange is to receive your partner's energy all the way into your core; close is not sufficient.  Bring it inside of you, to the various places where you have stimulated sexual energy, and blend.  Resolve into light, within your deep-center/s.

The major areas of exchanging sexual energy that I'll cover are:

1) No-technique.

2) Upper gate.

3) Lower gate.

4) Field level.

The first technique that I want to acknowledge, briefly, is no-technique.  Lots of healthy exchange of sexual energy happens spontaneously, any time, with no special effort, especially when things are "feeling just right" between the two of you.  I just want to acknowledge that, and pay reverence to it.

The "upper gate", here, refers to the mouth and eyes.

The mouth.  The conception vessel (the part of the microcosmic orbit that runs down the front) jumps the gap, in the mouth, from the upper palate to the lower palate. There are a couple of easy ways to tap a tingle with this:

        The first is to run your tongue under your partner's upper lip.  A bit of an odd gesture, but the zing is worth at least a brief experiment.

        The second is to simply put your attention a ways back on the upper palate of your partner.  This would be, approximately, back to the soft palate; the conception vessel flows strongly there.  This simple gesture of attention connects your partner's conception vessel with your own.  The conception vessel will be re-visited at a corresponding place within the lower gate: the g-spot.

Sharing breath is another way to share through the upper gate.  During intercourse, sexual energy floods your entire system, as well as your breath. Try this simple way of sharing:

        Have your partner inhale, all the way down to the sexual center.  Then put your mouth over your partner's, and receive your partner's exhale with your inhale. Inhale all the way down to your sexual center, and hold your breath for a little while - in a very relaxed way, letting your sexual energy mix with the energy that your partner has given you, through the breath.

Swallowing your partner's saliva is something that is commonly recommended in classic Taoist texts.  You can get a feel for gently swallowing - then guiding the feeling of - the saliva all the way into your lower tan tien.  This skill is especially produced by the practice of Yi Swallows Chi.

I refer you to SimonV's post, which I think captures so well the spirit of exchange in the lower gate. You can easily adapt the concepts for meditations by either partner.

Additionally, Turtle Breathing, specifically for men.

Some areas for you and your partner to meditate upon, along these lines:

testicles

sperm palace

tip of the penis

g-spot

ovarian palace

uterus

By "field level", I kind of mean sharing in a broader way (verses the specificity of the upper & lower gate exchanges, above).  While it may emphasize some part or another, there is a more over-all sense of body-sharing.  This often happens naturally, through the skin, but there are also specific helpful techniques.

This completes the tour of some major areas of exchange.

For men...

Exclusive focus on conservation of substance (semen retention), without the presence of exchange of energy, tends to lead a man towards energetic isolation, particularly in the lower jiao... which not only cuts off the possibility of alchemical balance, but it also diminishes love and intimacy during intercourse.

The adoption of exchange re-directs a man's focus towards sharing experience.  There is still appropriate retreat and inward consolidation, at times, but it's now balanced by equally developed practices of opening and exchanging.

As well, one's attention naturally is drawn towards one's partner's sexual response cycle - by a blend of care for your partner and the joy of receiving effectively.  Nurturing, and then riding, waves in endless variations of expression... now benefits the both of you.  With exchange, the experience of esoteric intercourse as a shared experience becomes possible.

There are many benefits to exchange.  It's really the draw, the alchemical promise and hope, that brought you and your partner together for intimacy: the joy of union, in such a way that it restores the center in each of you.

All that said, its important to have appropriate time & space for solo integration, to stabilize within the vertical alignment of your own deep-centers, and to refine personal energetic patterns into Light.

In the practice of the Southern school, even if one can obtain elixir from one's partner, one must then resume the practice of clear serenity, embrace the fundamental, preserve unity, return to emptiness, and go back to nothingness; only thus can one achieve final settlement.  Otherwise, hopes of attaining reality will vanish in the end.

                                - from Vitality, Energy, Spirit : A Taoist Sourcebook pg xvii

There are a number of topics that dove-tail together with the exchange of sexual energy, that are beyond the scope of this essay (or are covered sufficiently in other sources).

Awareness

Exchange is not just about energy, its about awareness. (Seems too obvious to state, I know.) This brings up all sorts of other topics; the most basic of which is to simply be aware of your partner.

Read the important Elixir Theory section in the essay Balancing Male Sexual Energy in the Lower Tan Tien, on this website.

The word, "exchange", implies both giving and receiving; these practices work best when practiced by both partners.  They are, in fact, alchemically counter-productive when used with insincerity.  Read the important Sincerity section in the essay Balancing Male Sexual Energy in the Lower Tan Tien.

The valley orgasm is talked about in the Healing Tao books (chapters 9 & 11 in the male book, chapter 8 in the female book).  There are a number of topics that have gotten lumped together in the term, "valley orgasm":

        varying pace - The slower cycles can be used to relax inwardly, which facilitates the de-localization of sexual energy, and can provide time - and the inner open spaces - for a variety of alchemy.

        open in the deep-center - the relaxed internal space needed for alchemy to occur, and the medicines that sustain the open center vis-a-vis sexual energy: connected breathing, opposite gender's energy, heart energy, and heavenly energy.

        whole body orgasms - an option, and alchemy can also occur satisfactorily in one area or another.

        upward orgasms.