There's no secret about it, somen, hiyamugi, and udon are all Japanese noodles made from the same ingredients of flour and salt water, practically made in the same manner when mass-produced. Only thickness differentiates any type from the other two, according to the Japanese Agricultural Standards (JAS):

somen: less than 1.3 mm in width
hiyamugi: 1.3 - 1.7 mm in width
udon: 1.7 mm and above

Somen basically look and taste like angel hair noodles. Served cold in somen tsuyu soup, it is a popular summer dish. A variety of toppings exist, which may include but are not limited to nori seaweed, roasted sesame seeds, cucumber slices, chopped negi, stir-fried egg slices, ham slices, ginger, ground wasabi, and tomato slices. (However, traditional somen aren't supposed to have toppings like hiyamugi.)

Nagashi somen
Participants carry around a bowl with soup and toppings, waiting at a bamboo waterslide. One person serves the prepared cold somen noodles from the top of the bamboo waterslide by releasing small amounts at a time, while participants scoop out their share from the slide with chopsticks.