https://www.youtube.com/watch?v=qci2Qgi7VGE
https://www.youtube.com/watch?v=rPaKn7kD2EA
http://www.youtube.com/watch?v=n8JjLbSEUFw

After hot-end touches bed, Z stopped
It better use with auto bed leveling system in marlin firmware.

My configuration for marlin

define X_PROBE_OFFSET_FROM_EXTRUDER 0

define Y_PROBE_OFFSET_FROM_EXTRUDER 0

define Z_PROBE_OFFSET_FROM_EXTRUDER 0.4