King's Tradition Chenrezig

[namoh]

Hi,
I was wondering about the lineage of the King's Tradition/Gyalpo Lug Chenrezig Empowerment that HH Sakya Trizin gave in several places on his last visit to the US. It appears to come from King Trisong Deutsen, so is this actually another practice from Kama that the Sakya have upheld (like Phurba) or is this a practice that found it's way into the Sakya tradition later on?

Much thanks!