As can be seen in the photo above if you want to thread one end of this line down through itself you might run into a space problem.
The issue is that the outer braid works like a finger puzzle and it tightens down on the inner braid to protect it and stop it from slipping through the outer as it is pulled tight.
You can release this hold and give yourself some “room” by “Milking” the outer down and opening it up like you would a finger puzzle. BUT there is only so much room for this as if the outer is too big it won’t grip the inner with enough force to create a strong bind.
Therefore, when you are splicing you are asking the outer to have 2 X the inner run down inside it and things get a bit tight.
You can’t thin out the line other than at the bitter end (the end you are “tailing” back down|), so you have to make sure you taper that, thin it enough, to allow it to be pulled down into the outer, but not enough to allow it to be too loose a splice.
Its an art and with a bit of practice you will find the right balance of threads to remove as you are tapering for different lines. If you find it just won’t “tail” then go back and taper some more, but be careful as you can’t undo a taper. Start tighter than you might want and ease into it.