I've spent the past week implementing this feature I call push/pull toolchange. 

It started when @heinz posted his findings with the Cetus2 nozzle and its strategy for fast toolchanges:

The shownotes of implementation went something like this:

• Day 1
  • Write initial pseudocode
• Day 2
  • Look inside tool_change(...)
  • Write config modifications, looking at SWITCHING_TOOLHEAD for naming conventions
  • Write more detailed pseudocode
  • Try and understand what COLOR_A_MASK means
• Day 3
  • Look at how the extruder directions are applied (and ask if there's any specific thing I'm supposed to do so that I don't break everything, since this is outside the mixing-related code.
  • Look up what do { } while(0) does, and it seems to make sure if-blocks don't break things.
• Day 4
  • Find out there's something called MIXER_AUTORETRACT_TOOL
  • Understand a few other functions in mixing.h
  • Create a pushpull type
• Day 5
  • Turn off VSCode Cpp formatting so that it doesn't mess with the formatting that's already present
  • discover "mix" and related functions, and enable them for push/pull
  • Write initial implementation for pushpull in the mixing.h side of things, such as creating MIXER_PUSHPULL_TOOL instead of a colour in the pushpull type.
• Day 6 (today)
  • Find out that there are bit-mask macros and use them to tidy up my code.
  • Try, fail, try some more to create the new E_APPLY_DIR macro.
  • Change get_extruder_bits(...) to e_dir(...), where the former gave the thing that asked for it the entire 8 bits wheras the latter is a boolean on if that specific extruder is inverted or not. This made the MIXER_STEPPER_LOOP easier.
  • Allow pushpull to use the too_cold(...) function already in toolchange.cpp
  • Get more errors that stem from how I implemented the E_APPLY_DIR macro, as well as Mixer::pushpull
  • Finally get the E_APPLY_DIR macro to work by needing essentially a do { do { ... }while(0) }while(0)
  • Add a definition of pushpull_t Mixer::pushpull into mixing.c
  • Marlin compiles finally
  • Fix errors when optionals in config.h are disabled
  • Improve too-cold error message by adding an additional context message
  • Everything seems to work except I'm not getting any directions
  • Test to see that my bits are being set correctly

  • Find out that I need to apply the stepper directions, and I can't do that from mixing.h
  • Split the code so that the logic responsible for extruding was in toolchange.cpp.
  • Extruders work but there are a few quickly-squashed bugs.
  • Pull Request created.

Implementation estimate: 12 hours
Implementation time...
15 hours?

• Day 7 [Mar 31]
  • Code cleanup to make it look minty fresh.
  • Thought of a potential bug, confirmed and fixed it.
    • When going from a A5 B5 C0 D0 to an A0 B0 C5 D5 mix, the push/pull length was wrong as I assumed it would just be 2x all the time
  • Found out that G3 causes Marlin to crash and probably has something to do with the I and J values, 
    • Changing the HIJK mix characters to LMNO seems to fix it.