As default, Signin will be active & all other options are hidden & will get active when user will click on any given option.
Field will active on their appropriate option on basics of class="sign-up sign-in reset" each option is divide on these class basis. If user click on Reset Password option then only Email field will be shown & other fileds will be disabled, This will work on the basis of Class define with the option such as with Reset Password class="sign-up" is defiend.
When you clicked on any option either is signin or signup. Button text will also change through CSS. Below CSS code is used to change text on Button everytime when user click on any options.