When you expose film to light, some AgHal molecules converts to metal silver. When you put this film to fixer, all AgHal will be washed out, but these metal particles will not. They are not visible to naked eye, but can became centers of silver crystal forming, which will take place, if you process your film in physical developing solution, which contains silver nitrate and a developing agent. This technique is used when you need an extremely low grain, but sensitivity will fall in several times. Besides, it works badly with modern emulsions and if your developer or fixer contained strong silver solvent, these little metal silver particles possibly are already destroyed.