The one benefit of loop to loop connections that I like most is the ability to jump more than the standard 2-3 thousandths in diameter.
I think you are referring to the "rule" that the step down in hand tied leaders diameters "should" be no more that 0.002". I guess it is time for some myth busting.
That really makes no logical sense.
For example, a decrease in diameter from 0.020" to 0.018" at the leader butt is a 10% decrease in diameter but a decrease in 0.008" to 0.006" for the tippet is a 33% decrease in diameter. Why can is the butt section of a leader limited to a 10% change but the tippet section can be a 3.3 times as much of a change? That is totally bogus!
If there is any "rule" that is based on science, it would be based on a % decrease in linear mass. Just like a fly line tapers, so does a leader. It is the decrease in fly line mass as it tapers that rules over the fly line and the same is true with a tapered leader. The tapering of the leader results in a decrease in linear mass and that results in the preservation of forward velocity of the cast due to the law of the preservation of KE (kinetic energy).
Both a smooth transition of energy and an undetectable delivery require tapering. The reason is that energy, in the form of both the mass and velocity of the leader, is what delivers the fly. Tapering creates a system where a greater mass and therefore greater energy turns over lesser mass. An abrupt change in diameter (mass) from fly line to leader creates a hinge effect in which a low mass leader segment cannot accept the energy transfer at the end of the fly line/leader.
There must be a maximum % diameter change that can be made without resulting in a hinge effect. The only way to determine the maximum % change is by experimentation.
The experiment is based on the concept that what changes at any any transition point is the linear mass density of the monofilament. When we go from a thicker (greater mass) section of leader material to a thinner (lesser mass) section, we are decreasing (changing) the mass; and there is maximum limit to how much that mass can change and still get a smooth transmission of energy from one section to the next.
This experiment was performed by Gary Borger. He found that a 1/3 diameter change was the most that could be made without creating the hinge effect. This is close to a 50% change in linear mass density.
A 30% change in diameter = a 50% change in mass. The linear mass density of a leader varies with the cross sectional area, and this varies with the square of the diameter. So if we go from a 0.010" to a 0.007" the ratio of the masses is (7/10) squared or 49/100. The linear mass density of the 4X mono is about 50% of the 1X mono. Any greater change and Gary discovered that you will not get a leader that casts smoothly.
Although we didn't mention velocity specifically in the 1/3 formula, velocity is an inherent component of energy that is transmitted. When we said that 1/3 is the maximum change that the allows for as smooth transmission of energy, we were saying that the velocity cannot change enough to compensate for a diameter change of greater than 1/3. The two components of kinetic energy are always interrelated. For kinetic energy to be transmitted down a fly line or leader, a change in mass results in a change in velocity.
Gary writes: "To make the math simple enough for mental on-the-water work, eliminate the decimals. In other words, a diameter like .009" (2X) would be thought of simply as "9." Then, take that number, multiply it by 2, and then divide it by 3 to get the next smaller size you can use. For example, 9 * 2 = 18. Then, 18 / 3 = 6. The number "6" corresponds to .006" (5X). Once you get "multiply by two, divide by three" going in your head, you can do one third step-down calculations with speed and ease." So it is perfectly acceptable to tie 5X to 2X without having a section of 3X or 4X material between them.
Of course you can use a smaller difference since the 1/3 change is the maximum. So if the leader end is 4X but you want a 5X leader, you can tie on 5X to the 4X.
We must assume that the monofilament sections have the same stiffness for a 1/3 ratio. A limp section of mono cannot transmit as much energy forward. This is the principle on which the George Harvey leader is built. A limp section of mono is less efficient at transmitting kinetic energy than a stiff section of mono and the section of the leader with the limp mono collapses for a drag free drift.