Jason,
The 400 will down shift on its own if it sees the vacuum drop from the carburetor. The modulator and governor control normal day to day operations in the transmission. They will work with out a switch hooked up, but if racing or towing, you should have one.
The kick down switch will override the governor and downshift the trans any time the switch is tripped. The kick down switch also raises the hydraulic pressure so under a hard shift the clutch packs don't slip plus it holds the trans in gear longer for more power. You will notice at full throttle shifts that the rpm is way to low with out the switch.
Joe
