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It's called Second Impact Syndrome. Unfortunately it is not one of those rumors you hear about on the net. It's real and well documented. You need to wait at least a couple of weeks until you ride again.
I pulled the following out of "Clinics in Sports Medicine", July 2003
"What Saunders and Harbaugh [21] called the second impact syndrome of catastrophic head injury in 1984 was first described by Schneider [22] in 1973. The syndrome occurs when an athlete who sustains a head injury—often a concussion or worse injury, such as a cerebral contusion—sustains a second head injury before symptoms associated with the first have cleared.
Typically, the athlete suffers postconcussional symptoms after the first head injury. These may include visual, motor, or sensory changes, and difficulty with thought and memory processes. Before these symptoms resolve—which may take days or weeks—the athlete returns to competition and receives a second blow to the head.
The second blow may be remarkably minor, perhaps involving a blow to the chest, side, or back that merely snaps the athlete's head and imparts accelerative forces to the brain. Sometimes this second impact is unrecognized among the multiple hits that occur in each game of football. The athlete may appear stunned, but usually does not lose consciousness and often completes the play. The athlete usually remains on his feet for 15 seconds to a minute or so but seems dazed, like someone suffering from a grade 1 concussion without loss of consciousness [19] . Often the athlete remains on the playing field and sometimes walks off under his own power.
What happens in the next 15 seconds to several minutes sets this syndrome apart from a concussion or even a subdural hematoma. Usually within seconds to minutes of the second impact, the athlete, conscious though stunned, quite precipitously collapses to the ground, semicomatose with rapidly dilating pupils, loss of eye movement, and evidence of respiratory failure.
The pathophysiology of SIS is thought to involve a loss of autoregulation of the brain's blood supply. This loss of autoregulation leads to vascular engorgement within the cranium, which in turn markedly increases intracranial pressure and leads to herniation either of the medial surface (uncus) of the temporal lobe, or lobes below the tentorium, or the cerebellar tonsils through the foramen magnum. The usual time from second impact to brainstem failure is rapid—2 to 5 minutes. Once brain herniation and brainstem compromise occur, coma, ocular involvement, and respiratory failure ensue precipitously. This demise occurs far more rapidly than that usually seen with an epidural hematoma. This precipitous deterioration was seen in each of the three cases above."
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