How to extend the counter-part sign? What even is this?

I am a practitioner of tummo and I can say that after I after entering jhana multiple times can enter jhana quicker than I used to.

The best advice I can give to anyone is relaxed awareness in your meditation object! This is how I gained proficiency!

Gaze into space with both eyes, with the body motionless, without speaking, and with a gentle and natural coming and going of the breath. Tilopa has said: Hold the mind as if it is space.Make it like space and bind the breath evenly. When there is complete knowledge of equality, it dissolves completely.

You can different exercises in order to gain proficiency like changing your object of meditation size,colors ect...

Also take a look at this

I think i found some insight into so i want to provide my own answer for others. This is just one view though.

Pure, Beautiful mental image. It will feel strange at first. But mentally joyful. We never experienced such things but the mind will try to described & understand it. For some its some form of light, some a ecstasy feeling. But all 5 sense are gone. You see no light you feel no sensations but this is just the mind trying to understand & classify the beauty. Even though the mind experience different perspectives one thing is the same the pure mental object.

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".