Two conditions to getting one time use points are that you can only do it at 7 months out or less and you do not have the needed points left in the "current" use year applicable to the trip. It is promoted as something you can do to prevent having to borrow from a future use year and thus you do not need to borrow points from a future use year first before getting the one time use points. Note that you are also not required to bank points first to be able to use it.
Example: you have a Feb use year with 100 points and today you want to reserve early Feb 2016, 7 months out and need 114 points for that trip. With only 100 points in your Feb 2016 use year, you can reserve that trip and buy 14 one time use points. That is true even if you have 100 points in your Feb 2017 use year and 100 points still left in your Feb 2015 use year that have not yet been banked. Of course, in the usual case you would likely want to bank those 2015 points and use them for that trip but that is not required before buying one time use points. All that is required is that you are using up any points left (i.e., points not already devoted to other reservations) in that Feb 2016 use year during which your trip will occur.
Note that my understanding appears to be different from what is mentioned above that you can buy the points while still leaving points in your current use year applicable to the trip. This came up before on these boards and others who had tried reported MS has not allowed that. In other words, in the example above, where you have 100 use year points in the applicable use year and need 14, you cannot just use 90 of your points and buy the 24 maximum one time use year points while leaving 10 points in your use year applicable to the trip.