Please can u explain the following massla regarding sleep…

Respected Mufti, it is stated in Nur al-Idaah that your wudu is not nullified if you sleep in such a position in which your buttocks remain firmly established on the earth. I am also aware of the ruling which states that if a person is asleep whilst leaning against an object which, if removed, would cause him to fall then this invalidates his wudu. I understand this, however further on in Nur al-Idaah it says : (under the chapter explaining those situations which do not nullify wudu) , " One whose buttocks are established upon the earth even though he is leaning upon an object which, if removed, would cause him to fall." I would like to know the meaning of this because to me this sentence means that even if I am sleeping quite comfortably in a sitting position on a settee/ reclining chair my wudu is still intact. Yet I am v. sure that my understanding is quite deficient. The ref for the above quotation is: Mahmood. Z. (2000) The Book Of Tahara, Leicester, Inter-Islam Publishing company. (p.36)