There are, I suppose, three primary ways to breed a female. 1) Put a female in heat and a male together and just let Mother Nature run it's course. This works pretty well for strays, I am assuming here, and many "backyard breeders". 2) The Old School way: put them together on days 10, 12, and 14. This too has worked for many breeders for many years, but not all females ovulate at this point in their cycle and using this method can result in small litters or missed breedings all together. 3) Use progesterone testing. This is the only method which scientifically quantifies when and what is actually happening in the female's body, which tells the breeder what to expect and when to expect it. As you may have guessed, we use option three: Progesterone Testing.
Why is this important? We have two main factors involved. Egg and Sperm viability. Some females "run fast" and ovulate before day 10. Some "run slow" and ovulate well after day 10. The canine egg takes forty-eight to sixty hours after ovulation to be ready for conception, and remains viable for up to three days. In all from ovulation you have about five days. So knowing when this window of opportunity is open is key. In particular when you know it takes Sperm time to capacitate and have they a finite window of viability. Generally, sperm can be expected to be viable in the female up to seven days for a live mating. For chilled and frozen semen the timeline is much tighter with sperm viability being around 24 hours for chilled and only 12 hours for frozen.
Like two ships at sea wishing to exchange passengers while remaining in motion, there is an optimal time for this to happen, the further we get away from the optimal window of opportunity the lower the odds of success. As you might imagine, just like the passenger exchange. Too early, not all of them will make it, too late you get the same effect, but for different reasons. In the sweet spot as many that could make it will. So having everything in place at the right time really matters.
I'm just a guy suffering with an infatuation with gundogs since childhood. Forty some odd years later this is what you get.