how is 4ch spam protected?

short question: how do you protect your board from automated bot spamming? i cant see any captcha! normally, from my expirience, a board  would  be instantly spammed to death without captcha or such protective measures. but 4ch board seems to be free of those problems... how do you do that ?

<span style="display: none;width: 0px"><input name="email" type="text" /> DON'T TYPE STUFF HERE</span>