the short, tl;dr explanation

For conventional readings, you pick random cards in order to find a message, an answer in them – an answer you hadn’t yet found (or at least been unable to put into words) without the help of the cards.
For affirmative readings you deliberately pick those cards that affirm an answer you already have put into words, and know to be true, in order to visualise this already known answer through the cards. The benefit? Visualising that which you know to be true can do wonders if you have been having destructive self-doubts, or problems with taking the appropriate measures – e.g. because of a lack of courage, motivation, or energy.
An example of an affirmative reading I give at the end of this post.

the long explanation

Sometimes when we read the cards, the reading brings up something totally unexpected – completely new thoughts. Much more often, a readings speaks of things that, while they mightn’t have been expressed that way, we were still conscious of in a pre-verbal sense. And every now and again, a reading is glaringly obvious, affirming exactly what we already know to be true, what we’ve already told ourselves a dozen times. Now, one might think that these obvious readings would be the least helpful. Funnily enough, the opposite has been true for me. And it might be true for you, too. Because sometimes, what we need from the cards is not advice, not a solution. We already know what to do, we’re intellectually aware of the solution. We just can’t motivate ourselves to act accordingly; we can’t bring up the courage or willpower or energy to implement the solution. I personally suspect that this happens especially if self-doubts get in the way (“I believe I know what’s right, but what if I really don’t?”), or if, for whatever reason, we suppress or don’t find access to the emotions that should naturally accompany our situation, or, if the situation causes disproportionately strong or misplaced emotions.
In such cases, what we really need might be an affirmative reading: an affirmation of the answer we already have. Reading cards in the conventional way – picking random cards and looking for an answer in them – might give us such an affirmative reading by chance. But if an affirmative reading is what we want, we might as well go for it directly by deliberately picking cards that affirm the answer we know to be true, the solution we are already aware of.
Having a good, long look (literally) at what we know to be true has great potential to:

  • self-empower us by affirming that we do in fact know what’s what (e.g. what is good for ourselves, what needs to be done, etc.)
  • elicit previously suppressed or muted emotions that are not just appropriate for the situation but also necessary for acting appropriately
  • help us distance ourselves from disproportionate or misplaced emotions which have been keeping us from acting appropriately
  • help us stick to our guns even when there’s opposition
  • etc.

an example

D. is in a relationship they are aware is borderline abusive and has been slowly deteriorating further. They also know they should break up with their partner and move on, before the situation becomes worse, but somehow they’ve never managed to take that step. So first, D. picks two cards that represent the relationship in its current state: it is already showing borderline-abusive characteristics (Whip), and is well on its way to deteriorate (Mice) into even darker places.

This visual representation of the relationship helps D. to affirm to themselves that yes, it really is that bad! Looking at the two cards drives home the message so clearly that D. now finally starts to feel afraid; caged – feelings very appropriate for the situation, which had nevertheless not come up when D. was only thinking about the situation, not looking at it from the outside as is the case now.
D. quickly adds the two cards which represent the solution they know to be true: the Scythe (for the break-up), and the Ship (for the moving on part after the break-up).

Now that the solution is visualised as well, D. feels immediate relief, and a strong sense of freedom and possibilities. Hopefully, having felt these positive effects of implementing the solution will motivate D. to finally go through with it.