The dimple is not only a visual embellishment to your finished knot but also serves to ensure the knot is tight and requires virtually no adjustments once tied.
As with all elements of style, you must be practiced enough to know what you’re doing all the while having your appearance seem effortless and natural. So don’t believe those who are baffled by the dimple saying such things as “Sometimes it just happens” as if it’s the work of the gods.
We are beginning this discussion at the point where we have pulled the wide end of the tie up and through the back of the loop and are currently guiding the tip of the blade through the front to finish off our four-in-hand. This is the moment where it can all go terribly wrong.
In my experience the key to achieving this final adornment is to position the thumbs below the knot under either side of the top blade with your forefingers on the top side whilst guiding the blade downwards. You should now see the top blade start to buckle creating the dimple. Then with the tips of the thumb and forefinger of one hand and the other hand holding the bottom blade in place make the final adjustments.
How do you know if you have produced a taut knot? An easy way is to view your finished work from the side and see if your knot springs out from the collar with a pleasant arch.
I have included this YouTube video link which I feel is the closest approximation.