As stated, they do not go into the special 'holding' account but are returned to the year into which they were borrowed.
If by chance you have any other reservations made for the same use year that used regular points, sometimes you can have the 'borrowed' points returned from the cancelled reservation reallocated to the other reservation, thus freeing up 'regular' points which in turn could be banked.
Whether you can do this or not depends on home resorts, the 11/7 month window used for the other reservation, or possibly current resort availability.
Example: You own 300 BWV points with a February UY. You make a reservation for September using 200 points. You make another 200 point reservation for December using the remaining 100 points plus 100 borrowed points.
July you need to cancel the December reservation. The 200 points are returned to your 2006 UY account as 100 'regular' and 100 'borrowed' points. You have MS reallocate the 100 'borrowed' points to the September reservation, thus freeing 100 'regular' points giving you 200 'regular' points in your 2006 UY. You can then bank all 200 points.
If you have multiple home resorts, then being able to do the above has some additional conditions. For example you make a reservation at 11-months for a home resort "A", and another reservation at home resort "B" using borrowed points. You cancel the 2nd reservation at resort "B". You can only reallocate those borrowed points to the reservation at resort "A" provided that (1) it's within the 7-month window, and (2) resort "A" has current availability. (Essentially you must cancel that reservation and rebook it using the points for the non-home resort)