The following form changes the content if the option "other" is chosen from the drop-down list, and scripting is available and enabled. The form doesn't submit anything; it's just provided for test purposes.
This technique is supposed to help with information overload, using progressive enhancement to only display form controls when necessary; if scripting isn't enabled, the "other" text box is always displayed. The problem with this technique is that the behaviour is unexpected, and could be missed, particularly on large forms with several controls. This technique is in violation of guideline 3.2, level 2 success criteria 3:
Changing the setting of any input field does not automatically cause a change of context.
Back to techniques for guideline 3.2.