This special feature is experimental for the stepper's drivers in standalone mode (Creality 4.2.2, 4.2.7, F1, F4) and is here for your test. It is possible to enable it by editing the configuration file, of course, Linear Advance can be used normally in boards where it used to not work before. LA may have Classic Jerk and S-Curve Acceleration disabled and Junction Deviation enabled.

Under default conditions, extruder axis movement is treated in the same way as the XYZ linear axes. The extruder motor moves in linear proportion to all the other motors, maintaining exactly the same acceleration profile and start/stop points. But an extruder is not a linear system, so this approach leads, most obviously, to extra material being extruded at the end of each linear movement.

Take the common test-cube as an example. Even with the best tuning the corners are usually not sharp, but bleed out. The top solid infill displays roughness where the print direction changes on perimeters. These problems are minor or even imperceptible at low printing speeds, but they become more noticeable and problematic as print speeds increase.

Tuning the flow can help, but this may lead to under-extrusion when starting new lines. Some slicers include an option to end extrusion early in each move, but this adds more complexity to the G-code and has to be retuned for different temperatures and materials.

Since the root cause is pressure, LIN_ADVANCE de-couples extrusion from the other axes to produce the correct pressure inside the nozzle, adapting to the printing speed. Once Linear Advance is properly tuned, bleeding edges and rough solid infill should be nearly eliminated.

From Prusa site. Calibration G-code printed with PLA at 215 °C. Most consistent line achieved with K-value 0.05 (green square).

• Better dimensional precision due to reduced bleeding edges.
• Higher printing speeds are possible without any loss of print quality - as long as your extruder can handle the needed speed changes.
• Visible and tangible print quality is increased even at lower printing speeds.
• No need for high acceleration and jerk values to get sharp edges.

• Some slicers have options to control the nozzle pressure. Common names are: Pressure advance, Coast at end, extra restart length after retract. Disable these options as they will interfere with Linear Advance.
• Also disable options like wipe while retract or combing. There should be almost no ooze, once the proper K-Factor is found.
• Recheck retraction distance, once Linear Advance is calibrated and working well. It may even be as low as 0, since pressure control reduces the material pressure at the end of a line to nearly zero.
• Don't forget to do the Calibration

The transition between Slow Printing Speed phases and Fast Printing Speed phases are the points of interest to determine the best matching K-Factor. Following illustration shows a magnified view of a line where the K-Factor is too low: