shifting the 4L60 manually...
itll wear out the clutch packs from the stress on decel. 4l60 trannies are weak when not built with the best parts..or with high mileage. you may get away with it for a while..but i wouldn't try it just my $.02
you can still downshift manually, but the lower your speed, the less it hurts it. you will learn at what speed each gear can take, like say i wouldnt down shift mine into second at much more than 25 mph, and first must be under 10. lol of course mine will do about 75 in second gear but u wouldnt want to down shift it then 
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its almost useless to upshift a 4l60e even more so to downshift the sunshell is a weak componet in our trannies and it does not take much to break the bearing retainer off of it.if you wanna shift get a 6-spd
Once I accidentally downshifted my V6 Camaro with 4L60E from 3rd into 2nd at about 70 mph. I intended to go to overdrive, but went the wrong way. 
From then on it slipped on the 2-3 shift under high rpm/load.
From then on it slipped on the 2-3 shift under high rpm/load. 
You would slow down faster, I can see using that if you need to and think quick enough to do it. That and slowing down with the Gears does sound bad ***, mine pops when it does that.
this is exactly the reason i bought 6speed cars. i like autos and all, they have more consistent launches. but i know i would be wanting to shift gears, its just instinctive to me.
