It seems that the so-called Muladhara chakra is located in that area. At first, I thought it might just be that I was meditating for too long and my buttocks were tired. However, even when I took breaks more frequently, when I resumed meditation, the area around my buttocks still pulsed with a heartbeat. It seems that this is not fatigue, but rather a "sign" that the energy flow has improved, which is indicated by the heartbeat. I think this is happening near the Muladhara chakra (base chakra).

I don't think there was no pulsation before, but when the energy flow improves like this, the pulsation becomes more intense, so it becomes easier to recognize it as a sensation. In terms of location, it is noticeable when sitting, and it pulsates quite constantly, so it may have become difficult to meditate for as long as before. I will observe it for now. It may be the same as before once I get used to it.

In particular, when I meditate and focus on the Ajna chakra between my eyebrows and draw energy from the Ajna chakra (from the universe?), the pulsation in the Muladhara (buttocks) becomes stronger in response. According to the writings of Professor Honzan, the Ajna chakra is directly connected to the Muladhara chakra. Indeed, I have that feeling.

Then, that pulsation and the energy of the sacrum become one, forming a solid foundation at the base of the body, and enveloping the entire body with energy.