The following text is by Adolph E. Peschke as presented in the 1998 printing of the 1993 edition of the Pioneering Merit Badge Pamphlet in the Lashing Section describing the traditional diagonal lashing…
When putting crossed braces on a structure to keep it from racking (as used when making a trestle), the most important lashing is the diagonal lashing where the spars cross.
When the cross spars are properly assembled on the trestle, they will be standing apart where they cross. That is, there will be a few inches of space between the spars where they cross at the center of the X. To pull them tightly together, a timber hitch is used to start the lashing. As the timber hitch is pulled tight, the spars are sprung together.
and in the Basic Knots Section describing the timber hitch:
The timber hitch is a knot that can be tied quickly. As strain is put on the rope, the knot gets tighter, yet it remains easy to untie.
To tie a timber hitch, first wrap the running end around the timber log or spar. Then loop the running end around the standing part of the rope, continuing to wrap the running end around itself a few more times. This will form a hitch that will tighten on the timber as the rope is pulled. After the timber is dragged or hoisted into position, the timber hitch is easy to untie.
A note about this final half hitch: when using the timber hitch to lift or pull an object, that added half hitch combined with the timber hitch forms what has been referred to as a Killick Hitch. John Sweet in Scout Pioneering suggests this combination when making a lobstick to throw a line over a branch. The Killick Hitch is also known as a Kelleg Hitch. The timber hitch is most always exampled as the first step in tying the combination.
In a diagonal lashing the wrapping turns cross the poles diagonally, hence its name. A diagonal lashing is used when there is a need to close a gap between two spars or when they spring apart, in other words, when we want to bind poles together where they cross each other but do not touch. This most commonly occurs when the ends of the spars are already lashed in place in a structure, as in forming the X-brace of a trestle. Both the traditional diagonal lashing and the Filipino Diagonal Lashing accomplish the task of drawing two spars together that do not touch. The traditional method uses a timber hitch to spring the spars together. The Filipino uses a lark’s head.*
Why are we so high on the Filipino Diagonal Lashing? Simple. It’s easier, much faster, and just as efficient. There are different methods to tying the Filipino Diagonal Lashing, but all have the same advantage of working both ends of the rope simultaneously. While wrapping, both ends move in exactly the same way which makes for quick work, and makes it easy to apply tight wrapping turns. Many who tie this lashing have adopted Gerald Findley’s clear-cut approach, providing welcome consistency across the board. After a little practice, tying it will become like second nature:
Halve the rope and place the bight formed in the middle behind an upper diagonal. Reeve the ends through the bight forming a lark’s head. (See 1. in the diagram below.)
Pull both ends tightly to the right, drawing the two poles together. (See 2.)
Begin the wraps by carrying both ends diagonally behind the poles around the opposite diagonal to the one where you started the Larks Head. Carry the ends over the front pole. At this this juncture, using both ends of the rope, you now have wrapped one complete turn. (See 3. and 4.) For added strength, you can take another turn, which would be comparable to four single wraps using the “traditional” method.
Whether taking two turns or one, to position the rope to wrap in the other diagonal, pass both ends behind the rear pole, pulling tightly. ** (See 5.)
Carry both ends in front of the poles around the other diagonal. Once again, for added strength, you can take another wrapping turn. When finished wrapping, pass the rope tightly behind the rear pole. (See 6.)
To begin the frapping turns, separate the ends and carry one over and one under the front pole. (See 7.) Note: The end that is carried over the top pole will be singularly over all the wraps which is is fine. This will position the ends to frap in opposite directions between the poles. (See 7.)
Take two or three tight frapping turns between the poles around the wraps. (See 8. and 9.)
Finish with a tight square knot. (See 10.)
This diagram illustrates only one (double) wrapping turn for each diagonal. Click on the image for a very clear, larger view.
* James Keller, Director of the Pioneering Area at the 2013 National Jamboree points out, by virtue of the way it’s started, the Mark II Square Lashing can also be used to spring together two spars.
** To avoid any possible confusion, in this instance, the rear pole refers to the one positioned farther away from you, and the front pole refers to the one positioned over the rear, and nearer to you.