Special warnings are available to the police and basically allow the officer to ask for an explanation for any objects or marks found on the suspect or found at the scene. So if the suspect is arrested for murder and is covered in blood and there is a knife at his feet a special warning would be appropriate.
The wording is normally messed up by interviewing officers. Its a repeat of the caution really and not much more than pointing out that there may be an adverse inferences drawn. A good interviewing officer will make it sound terribly serious. But if the advice was No Comment then the client should continue to answer no comment to all questions including the special warning.
Special Warnings come from the the Criminal Justice and Public Order Act 1994 sections 36 and 37 as well as PACE
10.10 When a suspect interviewed at a police station or authorised place of detention after arrest fails or refuses to answer certain questions, or to answer satisfactorily, after due warning, see Note 10F, a court or jury may draw such inferences as appear proper under the Criminal Justice and Public Order Act 1994, sections 36 and 37. Such inferences may only be drawn when:
a) the restriction on drawing adverse inferences from silence, see Annex C, does not apply; and
b) the suspect is arrested by a constable and fails or refuses to account for any objects, marks or substances, or marks on such objects found:
c) the arrested suspect was found by a constable at a place at or about the time the offence for which that officer has arrested them is alleged to have been committed, and the suspect fails or refuses to account for their presence there.
When the restriction on drawing adverse inferences from silence applies, the suspect may still be asked to account for any of the matters in b) or c) but the special warning described in paragraph 10.11 will not apply and must not be given.