The following construction can be found in one of the post in the ELU.
- remembering that the heart fibrils are what vibrate abnormally to cause a heart attack
The verb "to vibrate" is in the third person plural, and the subject pronoun has for antecedent a plural noun; this is apparently normal, but only apparently so (an innate notion of singular agreement impinges upon my mind, yet not in any exclusive fashion, the logic of a plural taking over incessantly); numerous existing cases show that given a plural antecedent, the agreement of the verb can be either plural or singular; cases "a" and "b" below, respectively, are some examples.
(ref. a1) some other role must be found for it. Motivating reasons are what is believed;
(ref. a2) Since the actual behaviours and decisions are what is of interest to us,
(ref. a3) The rules and principles that emerge from these sources are what is compendiously called constitutional doctrine.
(ref. a4) But rather they are what is nearest and least dangerous to the rioters.
(ref. b1) Listening and being heard are what are important.
(ref. b2) A workable definition of what we mean by 'drugs' has been used, albeit a circular one, so that drugs are what are usually included in the debate about drugs.
(ref. b3) Degrees of height are what are now referred to. The natural mind, by its two capacities called rationality and freedom, is in such a state as to be capable of ascending through three degrees
(ref. b4) The elements of design are what are actually on the film frame, such as points, lines, planes, shapes, and colors.
I cannot get at any source treating this case. There should be a clear-cut rule about this agreement, or so I imagine for now as I have not given this problem enough time to fathom any real intricacies. If so, what is it?