Instrumentation gets activated by the '-fdebugging' dynflag.

All the instrumentation occurrs in the desugarer; it consists of inserting 'breakpoint' combinators at a number of places in the AST, namely: 
 - Binding sites
 - Do-notation statements 
These 'breakpoint' combinators will later be further desugared (at DsExpr) into ___Jump functions.
For more info about this and all the ghci.debugger see the page at the GHC wiki:

http://hackage.haskell.org/trac/ghc/wiki/GhciDebugger