This, as others have noted, will make it many times more difficult to manipulate yields lower in the future as the "Fed is moving to a new way of looking at asset purchases."
As we explained in detail 15 months ago, Keeble notes the Fed appears to have clearly signaled that the degree of accomodation is not linked to size of the Fed balance sheet, but that the flow of Fed buying is "very important." So, it's the flow, not the stock.
Keeble does warn that rates could rise once again in the next several weeks if Congress reaches resolution to avoid government shutdown - which could perhaps be the post-fiscal-year-end signal for another Taper-tantrum.
All caps aside, what this means is simple: if it is indeed flow that matters (and it is), then Fed intervention can never stop, period. If the stock of a central banks' assets is irrelevant, the Fed can have $1 on the left side of the balance sheet or $1 quadrillion: it does not matter - if the market expects the Fed to stop buying assets tomorrow, then the crash is as good as here. That has precisely been the biggest flaw with the Fed-accepted stock model, per which Bernanke can buy up a few trillion in MBS and the stock market will be flat as a frozen lake. Alas, this is increasingly becoming obvious is not the case. Hence flow.