It increases line pressure so the valve body shifts faster.

From what i've read, a shift kit may actually help your tranny because it reduces friction-induced heat or something

Hey, that's why we're here! LOL. I don't understand the whole external thing, and the link really doesn't help. I know for sure that lots of people have the Star or Trans-Go shift improver kits. These are described like the thing you're looking at: more efficient and less of a performance item. I'd say go with one of those. They're not too expensive.

Can't remember exactly where I've seen them advertised but google Trans-Go Shift Kit and I'm sure you'll find their website.

Good luck.

 Basicly shift kits worked on a simple primiters,, they uped the lline pressure and you could
drill larger holes in seperator allowing  more volume to a clutch pk and block acculators in
their bores or stiffin the accullator springs...as they assume some of some of the oil
volume comeing from the shift valve... The 604 is a different type of automatic in comparrison
to general parts used in most automatics.. It was designed with no bands or sprags in order
to save length and size in order to make it fit where the preveious 3 speed auto would fit
to save staggering costs on changeing assembly line at the auto maker...The onboard computor
for one is going to adjust the shift how it wants , not how you want... you change the line pressure, it will simply lower the amount of oil to the shift, putting it back to the same old shift... I think a person could cause proublems for the 604 by upping line pressure due to
the issue of no sprags that is a big help for transmission of old as it will hold a drum
in one dirrection but allow it to free wheel the other, 604 shifts must be precice to relieve
one clutch pk while appling another without engine flares or tie ups between
shifts,, I dont feel their is any aftermarket shift kit that compliments the 604 at this  time.
Roger Reynolds ,,Hannibal transmission and Roadrage quick change gears...

don't install the venom 400, it's useless.

A second  reply on the shift kit,, I was myself thinking a external  shift kit would be agood
Idea , I clicked on this site for one but It diddnt come up... I have been purchaseijg
stuff from radio shack and patching it in the wire harness and firming up shifts for the
chevy truck4l60 e unit, Il ldo some investigateing when I get time and basicly it allows you
to up the line pressure at the fllip of a switch and thismay be ok if it was used on
ocassion since if its used all the time the computor will eventually change the volume
index going to the clutch pk and return it to its idea of a shift... RREYNOLD@NEMONET.COM 
roadrage quick change gears   . P.S. If anyone knows of or how th Get ahold  or talks to
Robet at ERT, he  owes me monie,,,He wont answer my calls...