I've always loathed this 'concept', primarily because of it's name. I believe it's a bad trend to take commonly used words and turn them into a concept (e.g., "but", "like a", "work").

To me, 'but' is a case-by-case situation, with a few generalizations:

1. Replace the final centers cast off 3/4 with the centers doing the given call (TALLY HO BUT)
2. Everybody replace the last part of the call with the given call (LINE TO LINE BUT, SPIN THE PULLEY BUT)
3. Centers replace the last part of the call with the given call. So, then, I'm never sure what the caller wants for a NUCLEAR REACTION BUT. Does the caller want the centers to take the "but" call, or everyone? It's clear if the caller says "Nuclear Reaction But Centers Scoot Back" versus "Nuclear Reaction But Scoot Chain Thru".
4. Other instances:
   1. Flip Back But Centers Spin The Top
   2. Along But Circulate Twice
   3. Square the Bases But Outsides start a Split Dixie Sashay
   4. Chase Right But Boys Swing Thru
   5. Swing An Anchor But Flip Back (the centers replace the last TWO parts)
   6. Flippers Flow But Criss Cross It (the 'but' is part of the call name)
   7. Pass The Axle But Cast 1/4

I think of 'but' as a tool that the caller can use ANYWHERE he/she likes anywhere within the call. Evidence of this can be seen even at lower-level dances where the caller says "Chain Reaction But Star 1/2", or "Spin Chain & Exchange the Gears but Promenade in Single File",.

"But" is usually used as a version of "Replace".