Not so in the case of "understanding", in fact quite the reverse is true.
If you are in a mathematics lecture and a concept is being explained and you feel that you understand it - does that guarantee that you understand it?
If someone undergoes some heavy meditation practice and suddenly have the feeling that they have acheived some great understanding, - you know that "everything has been made clear and all my questions have been answered" sort of thing - does that imply that they have understood something?
This sensation we have that we associate with "understanding" is not the same as understanding. It is sometimes, in fact often, quite spurious and can quite obviously be triggered in the absence of any sort of subject matter.
In order to know that you have really understood the concept being explained in the maths lecture you would have to use behavioural method of seeing if you could do the exercises in the text book.
If the meditator is unable to say precisely what was made clear or which questions were answered and what those answers were then it is more likely that the sensation they had of understanding was quite spurious.
So if we reject the feeling of understanding as being what understanding really is, and we reject the behavioural definition - what is left? Nothing.
So if "understanding" is anything it is an ability that can only be defined in behavioural terms - any other sort of definition is meaningless
But I did not say that consciousness could be completely defined in behavioural terms, I said we should separate out the things that can and the things that can't. For the reasons given above I think that "understanding" can only be defined in behavioural terms.
Other things, sensations like pain, depression, elation, can only be defined ostensively (and before anybody gets started, that is not the same as saying that they are not reducible to physical entities).
But if we have a structural definition that made from behavioural and ostensive definitions and which is not circular - then we will have as good a definition of consciousness as we have for anything.