He's talking about a B&M "shift kit" not a B&M shifter.
Manually shifting the transmission allows you to hold it in the specified gear longer, without having to worry about up shifting into the next gear like you would just by leaving it in "D". It's a common practice when racing (and on the street for some) but it does shorten the life of your transmission doing the "1-2-1" shuffle. B&M does make a ratchet style shifter for this purpose, but It's built for a C4, not an AOD and I really don't know anyone who's got one in their Mustang.