Semen retention is actually a fairly simple technique, they overcomplicate it in eastern traditions.
You simply need to strengthen the muscles you use to hold back urine with. Clench them regularly, until they get strong enough to hold back ejaculation.
When you are having sex, as you reach the point of no return, you need to clench these muscles as strongly as you can. This will cause you to have a dry orgasm.
An added benefit, is that you will stay hard and can continue with intercourse, for much longer than first time round. If you practice this regularly, you can basically last for as long as you want or need to. It will also give you a stronger erection and if my ex is to be believed a larger penis, because there is more blood being retained by the blood vessels. Maybe that was just a subjective observation from her part, but it did seem bigger even from my perspective.
Anyways, that’s just the physical side.
The real benefits are spiritual as your sexual energies are subsumed and transmuted. Instead of exiting downwards and outwards, they turn around at the base of the spine and flow upwards along the spine and into the brain, feeding illumination.
This can feed your Kundalini Awakening and once the Kundalini is awakened, the flow can become permanent in a practice known as Urdhvareta. If you reach the stage of Urdhvareta as I have, no more semen exits your body, it is being sucked up into the spinal fluid and streams upwards into your brain.
All this is central to Tantra and for some reason a great secret with elaborate ways to hide the sheer simplicity of it, but it is something that will come to you naturally, if you practice it.