It's the origin of the myth. He was the last Templar Knight of import and was burnt at the stake on that particular day for not divulging the Templar secrets.
The Shroud of Turin also came from this time period as the man was wrapped in the Shroud soon after his torture.
As the legend goes, due to the substance used as a whitening agent in that time, the lactic acid that was produced by Jacques' body reacted in such a way as to leave a faint imprint of his body and face.
As Masonic legend goes, the Shroud of Turin is not that of Jesus Christ.