Internal power is a subtle energy that can be channeled to make one self stronger.
Li Hō is the first character shown to be using this power. He first demonstrated it in his second fight with Ueki. By watching Li Hō, Ueki was able to learn to channel his heavenly power. The limits of human power is unknown, but Li Hō was able to stop the sacred weapons of both Ueki and Hanon.
Ueki himself has been able to channel his own internal power. He first used it against Li Hō, but then lost control of it and was easily defeated. He is seen using it again in his fight against Baron , but it was ineffective. Near the end of the series, Ueki learns to channel his power into his sacred weapons to make them more powerful.
Though it has not been confirmed, it's highly unlikely that hellions possess such a power. It is also unnecessary since hellions have enhanced physical power to compensate for their lack of sacred weapons.