Removing the #6 check ball (the one in the "D" shaped pocket) allows the maximum amount of fluid flow to the kickdown servo and the firmest 1-2 shift. There are two schools of thought on this, one school says that's good and another says it's not so good; thus the difference in kit makers instructions.

When using a VB with rear band apply the maximum fluid flow often causes severe overlap on a manual 1-2 upshift since the rear band just can't release as quickly as the kickdown band applies. Leaving the ball in place and adjusting the size of the servo feed orifice makes for a good firm shift without the overlap. Also removing the ball and/or blocking the accumulator makes the trans shift so hard that it can shear the splines in the aluminum front pinion carrier.

Depending on the year there can also be check balls in the transfer plate, one slows the application of the rear clutch and the other slows the application of the rear servo, this is intended to prevent parts breakage and has no adverse effect on performance. I've never understood why some kit makers have you remove them, no advantage that I can see unless you're trying to rock a stuck car by rapidly shifting from "R" to "D".