That ritual was constructed by Crowley during the events of his awesome diary excerpt "John St. John," Liber DCCCLX. Several times in DCCCLX he refers to "cutting the cross of blood" as a way to affirm his intention of spiritual pursuits over material ones and that may have had influence over his inclusion of similar instruction in the Pyramidos ritual. He also, in that diary, utilizes extensively the asana of the Hanged Man from the Mem card, which, in one way or other, symbolizes self-sacrifice (not out of some false moral obligation but rather to one's Will toward the HGA), and whose form is of the inverted cross of the crucifixion. I would read that diary for hints if more information is not forthcoming from others more learned than I.
"And they that read the book and debated thereon passed into the desolate land of Barren Words. And they that sealed up the book into their blood were the chosen of Adonai, and the Thought of Adonai was a Word and a Deed; and they abode in the Land that the far-off travellers call Naught."
- LXV 5:59