The computer will still down shift. If I set manual shift mode to 3rd gear it will go to 3rd gear, but if the computer detects the vehicle needs 2nd gear, it will automatically down shift to 2nd even though I have 3rd selected, when I let off the gas it upshifts back to 3rd. If I keep it set to 3rd it will never upshift higher than 3rd gear; same for all gears, stopping at the gear I select as highest gear used.

TEST #1
I set manual mode 5th gear at a slow speed and it went to gear 5 until I stepped on the gas more and it down shifted 3 times to 2nd gear leaving the indicated gear shown as 5th. When I took my foot off the gas it returned to 5th gear (I was counting gears as it shifted).

TEST #2
Then I set it to 1st gear. It never left 1st gear. (I didn't risk redline but read it might upshift to protect the engine if you reach redline)

It definitely holds a lower set gear and lets me force a down shift on the fly.

I found it useful in these situations.
#1 Lower gears for control or when going down a steep hill.
#2 I can manually down shift before using heavy throttle to have full engine power ready the moment I want it.
#3 I set 2nd or 1st gear at traffic lights to hold a lower gear for smoother take offs, then slide back to autodrive mode once rolling.

I can't force an upshift or force to hold a higher gear like I could on a true manual transmission. The computer wants higher rpms for harder throttle or it will down shift automatically.

I'm not even sure I'm using it correctly but it's working for me this way. I'd like to get more out of it if anyone knows more about the 4Runner specific shifter.

 

lol i thought you were complaining LOL

we had a Mazda 3 (station wagon) rental car one day, and i set it in 2nd gear (had the little paddle shifter thing like your 4runner) and took it to redline and it shifted to 3rd LOL, and it would also downshift when it felt the need