As you know you can add up to 9 x 0.05 lashes to a strong natural lash BUT you they don't have to be the same length. With the capping technique you can add, for example, one 4D fan that's 1mm longer than the natural lash and then apply a second 3D fan that is 2mm shorter on top. Make sure to graduate the bases so you don't get a T bar and make sure you are not overloading the lash.
Realistically, this is a technique that is best used for Lash Art projects or to add a hint of colur to a set. It is time consuming and you can now buy lashes that come in mixed diameter and lengths on one strip.
I capped this set with some Hot Pink 3D fans (D Curl) just to give a hint of colour. One of the benefits of capping in colour rather than just using coloured fans is that it can give a much more subtle look. When you look from underneath there is no break in the continuity of the black but the pink shows through just a little. It also gives a great effect when you look from above and then if you use a stronger curl for the capped lash, also looks impressive from the side.