He _does_ understand what the consequences alluded to are.

They are evidently that Lady Byron will speak out and tell her story.

He says she cannot do this till _after he is dead_, and then he shall not care.

In allusion to her accuracy as to dates and figures, he says: 'Be assured no power of figures can avail beyond the present' (life); and then ironically _advises_ her to _anticipate the period_,--i.e.to speak out while he is alive.
In Vol.VI.Letter 518, which Lord Byron wrote to Lady Byron, but did not send, he says: 'I burned your last note for two reasons,--firstly, because it was written in a style not very agreeable; and, secondly, because I wished to take your word without documents, which are the resources of worldly and suspicious people.' It would appear from this that there was a last letter of Lady Byron to her husband, which he did not think proper to keep on hand, or show to the 'initiated' with his usual unreserve; that this letter contained some kind of _pledge_ for which he preferred to take her word, _without documents_.
Each reader can imagine for himself what that _pledge_ might have been; but from the tenor of the three letters we should infer that it was a promise of silence for his lifetime, on _certain conditions_, and that the publication of the autobiography would violate those conditions, and make it her duty to speak out.
This celebrated autobiography forms so conspicuous a figure in the whole history, that the reader must have a full idea of it, as given by Byron himself, in Vol.IV.

Letter 344, to Murray:-- 'I gave to Moore, who is gone to Rome, my life in MS.,--in seventy- eight folio sheets, brought down to 1816.