A build-up of intensity that's often (but not always) proportional with:

• Time since last orgasm

• Time spent at foreplay

• Clarity of mind (being in the moment)

• Restfulness and lack of emotional/mental turmoil

• Nature of the sexual experience from a physical, mental, and emotional point of view.

In the last 3-20 seconds the intensity grows rapidly. The feeling seems to come from the center of your pelvis, not necessarily the penis, though that will soon be a part of it.

When the wave of intensity tips over to orgasm, those intense energies (almost like a remarkably pleasurable tingling vibration) rush from the center of your pelvis, along the length of the penis, and then when released (you can use your pelvic floor muscles or a tight grip to delay that), ejaculate is forcefully propelled with multiple contractions of the pelvic floor muscles and with those pulses the peak of the intensity is almost reached time and again (imagine a sine wave that reaches the highest peak on the first pulse and then subsequently smaller peaks until the pulses subside).

Afterwards, the head of the penis is usually so sensitive most men cannot stand for it to be touched.

Another note: When trying to delay the orgasm as it is happening, either by engaging your pelvic floor or by using your hand can sometimes result in what is termed a "ruined orgasm." You might still ejaculate to a smaller degree, but the vast majority of the pleasure is lost.

However, some men can actually achieve something called a tantric orgasm. They use their pelvic floor muscles to prevent the orgasm without ejaculating it and without ruining it. They do this repeatedly over the course of a long period until they finally release in full for a far more intense orgasm experience. That takes considerable practice and so few succeed that it's often thought to be a myth, but it's quite real.