The Mechanism in Plain Terms
Here is how it works. Neutrinos carry a tiny amount of mass. As the stream passes through a planet it picks up a trace of that planet's quality, and as it then passes through a newborn it leaves that trace behind. Which Gate the trace lands in depends on where the planet sat on the Rave Mandala at that moment. Repeat for all thirteen bodies, at two moments, and you have a chart. The image Ra Uru Hu used was two cars scraping past each other and each driving away with a streak of the other's paint.
Two Predictions
Two of the physical statements made in 1987 were ahead of the physics of the time. The first was that neutrinos have mass, which most physicists then doubted. In 1998 experiments in Japan showed that neutrinos change type as they travel, which is only possible if they have mass, and that result later earned a Nobel Prize. The second was the existence of magnetic monopoles, particles with a single magnetic pole; in 2009 researchers announced monopole-like behaviour inside certain crystals. Both arrived years after Ra had described them.
The Supernova of 1987
Human Design dates its own arrival to the same weeks as a rare astronomical event. In February 1987 a star in a neighbouring galaxy was seen to explode, the first supernova visible to the naked eye in nearly four centuries, and for a short time detectors on Earth registered a burst of neutrinos from it. Ra's encounter on Ibiza had begun in January of that year. The system reads the two events as one: a flood of stellar information arriving as the knowledge arrived.
Transits
The stream never stops, so the imprint is not the only thing it does. As the planets keep moving round the wheel they keep activating Gates, and Human Design reads those daily positions as transits: temporary activations that everyone on Earth feels at once, and that can briefly complete a Channel in your chart that is normally open. Your birth chart is the fixed pattern; the transit field is the weather moving across it.