Huxley's argument for epiphenomenalism grew out of a strictly physiological picture of the nervous system: if every bodily movement can in principle be traced to a prior physical cause, a nerve impulse, a muscle contraction, then there is no causal gap left over for a mental event to fill, and consciousness, however real as an experience, must be a side effect of the physical process rather than a link in its causal chain, exactly as the whistle of a steam engine is produced by the engine's workings without itself driving the pistons.
On this picture, a person's felt decision to raise an arm and the arm's actually rising are both effects of the same prior physical brain state, running in parallel rather than one causing the other, so that the feeling of having decided is, in a precise sense, an illusion about its own causal role even though the feeling itself is perfectly real. The theory's most serious internal difficulty is sometimes called the problem of epiphenomenal qualia: if consciousness does nothing, it becomes hard to explain why evolution would have produced it at all, since natural selection can only favor traits that make some causal difference to survival and reproduction, and a purely decorative byproduct with no causal role of its own is exactly what evolutionary explanation has the hardest time accounting for.