Nob,
The forcing cone is the rear section of the barrel just in front of the chamber where the shot charge enters the barrel proper, it is not the choke. The choke's up front at the end where the shot charge comes out, if there is any choke.
Lengthening the forcing cone gives the shot charge a slightly longer distance and angle before it's compressed into the bore diameter, and many people (including me) believe it helps a bit in perceived recoil.
Hans Vang, and others, make a fair amount of money in lengthening those forcing cones.
Denis