changes the printed output to a an easy-to-read form. It allows a readable output also when running a terminal based Mathematica session. Whether the result of FeynCalcForm[expr] is displayed or not, depends on the setting of $PrePrint. $PrePrint = FeynCalcForm forces displaying everything after applying FeynCalcForm. In order to change to the normal (internal) Mathematica OutputForm, do: ($PrePrint=.).