Code:

'/^Content-Type.+plain/{f=1;getline}

when line matched by "^Content-Type.+plain" is found, then next line is put in $0 by getline, and "f" is set to 1.

Code:

/^--/{f=0}

When line is matched by "^--", then "f" is set back to 0. So Those two commands set "f" variable to 1 for all lines between "^Content-Type.+plain" and "^--" in file. Next command:

Code:

/^(Subject|From)/||f{print}

checks if line contain "Subject" of "From" as first word, or that "f" variable is true (other than 0). If that is the case, then line is printed. So last command prints lines that were matched by "^(Subject|From)" or those between "^Content-Type.+plain" and "^--", becouse "f" variable was set to 1 for them.