MagneticReadHead is a bit different than what I think your parent post was asking about—that's more what Gallium was. The breakdown of approach is:

1. Gallium was a traditional gdb/lldb-style debugger

2. Debugger.jl is a pure interpreting debugger

3. MagneticReadHead works by compiling a modified version of the code you want to debug that is instrumented to add debugging hooks (this is different from how gdb/lldb work).