How to extend the counter-part sign? What even is this?
From my own meditation experience... For what it's worth...

Usually, when meditation is going well, you feel very comfortable and can relax, your level of concentration grows. Concentration on what? On nothing really, just on sitting and being in the moment. At some point even the breath stops being important, it's more like you no longer get distracted from sitting and staring at your own "raw" mind...

When that happens, it feels almost like you start tripping. I'm sorry for using a highly contemporary comparison, but I can't come up with a better image for what happens at that moment. At least in my experience, I start floating into some altered state of mind... While staying firmly in the present moment, without losing my concentration... In this state of mind I "see" feelings as "shapes" and "colors". It's hard to describe, because that "seeing" is not really visual, the shapes are not really shapes and the colors are not colors. They are feelings. But they are much more "loud" and "thick" than our normal feelings, they have more structure and differentiation so to speak. I believe this is the so-called "counterpart sign".

No two people have the same experiences, but when your get in touch with your deep subconscious emotional mind, you will experience something strange. This something, whatever it is in your case, will start small and then grow and expand. Then you have a choice to either keep relaxing and going deeper into that, or hold on, or come back out. In my experience, when I go deeper I eventually fall into something like a dream state.

In this dream state I lose sense of time, and I don't really have control, but there is still a sense of something going on, and I feel that, and it is good. Sometimes there is a feeling of some mental knots getting untied. Sometimes there is a feeling of resting. Sometimes there is a feeling of opening. Sometimes there is a feeling of sweetness. It also serves as a kind of reset button, meaning, when you "wake up" you feel much refreshed, more grounded, well-rested, balanced, and calm.

It's not really that magical. It's rather similar to a moment when you're falling asleep.

So my answer to your question, "how to extend the counterpart sign" is: "by relaxing into it, by relaxing deeeeep into the present moment".