p. 11 of GURPS Psionic Powers introduced this variation, also mentioned on page 11 of Power-Ups 8: Limitations.
as https://web.archive.org/web/20180103221155/https://dungeonfantastic.blogspot.ca/2013/09/all-out-concentrate-yes-but-with.html highlights, there is a thread on it at https://web.archive.org/web/20180103221126/http://forums.sjgames.com/showthread.php?t=117157 with a mention from Kromm
- One thing my players like is my house rule allowing All-Out Concentrate and All-Out Ready, sacrificing all defenses for +4 to the eventual task. Power Blow is at -5 after a turn. It can be worth spending that turn doing AOC to get +4 and make it -1, which is useful even at low skill. Personally, I find that more flavorful and truer to the source material . . . somebody who's squinting her eyes shut and making wax on, wax off motions with her hands should be defenseless while "charging up" the ability. But all of that is house rules and opinions only.