Tickle The Dragon's Tail Meaning

(physics) To bring two subcritical masses close together in order to find the edge of criticality.

Example:   Climbing so close to an erupting volcano was tickling the dragon's tail.‎
  Don't tell him the headline's misprinted; you don't want to tickle the dragon's tail.‎
  The laboratory head tickles the dragon's tail whenever we have guests as a show of machismo.‎
1957, Jim Harman, “Break a Leg”, Galaxy Magazine, volume 6, number 1-6, page 130:
  Every real boy likes to tickle the dragon's tail. Maybe I was a little old for it, but that's what I was doing there in the Moran III jungle. I got the shield off my cigarette lighter and jerked out the dinky little damper rods for the pile and started easing the two little bricks toward each other with the point of my lead pencil
1993, James Erwin Schevill, 5 Plays 5, page 52:
  PHYSICIST SLOTIN (manipulating the two hemispheres of the bomb at a table): Come closer. Watch me tickle the Dragon's tail
1995, George Robert Caron, ‎Charlotte E. Meares, Fire of a Thousand Suns: The George R. "Bob" Caron Story, page 219:
  And so it happened that Japan and the United States had also tickled the dragon's tail. The two subcritical masses called pride and cultural misunderstanding had been shoved together into one critical mass.
2003, Jonathan P. Binstock, ‎Jim Sanborn, Atomic Time: Pure Science and Seduction, page 13:
  Were this assembly functional, a person might manipulate the bricks of the radiant castlelike structure to slow down the neutrons, to "tickle the dragon's tail," as Morland writes