Flex lock-up/torque converter question

Kind of a random question, but can someone explain this to me?

I'm trying to better understand the S mode shifting vs D. I do understand S mode is simply a gear limiter. I get that.

But as I dug deeper, it's not a simple as that. There's more to it- and I think it might account for why some people think shifting is smoother throughout the stack when running in S mode.

As I interpret this chart- flex lock-up (is this the same as the torque converter?) isn't happening in 3rd and 4th gear when in S4. So... smoother shifting "feel" as you go through the gears?

So, what's actually happening here? Hoping someone can shed some light. Anything I say is pure speculation, as I know nothing about any of this. I just thought the chart was interesting and might bring to light something a lot of us didn't know before.

 

If you guys test drive a late model Tacoma you’ll be overjoyed to get back in the 4Runner. The Tacoma shifts way too soon and seems always locked from 3 on up, at any speed. My 4Runner does lug along in 5 as often as it can but the engine is more up to it. I suspect the converter is locked as soon as possible. Most every vehicle nowadays is like that. Kind of a shocker coming from a diesel pickup and an old 3spd auto TJ. The 4runner and especially the Tacoma would be much more enjoyable drive if that convertor wasn’t locked until 45-50 mph and never in 1-3.

 

Yeah I know exactly what you're describing here. If you think it's noticeable in the 4runner, it's like 10x as bad in the 3rd gen Tacoma. 2nd gen Tacoma is similar to 5th gen 4runners (very tolerable, imo).

The Tacoma guys call it a "stumble". And it's seemingly worse in some trucks than others. They actually issued a TSB for some of the 3rd gen Tacoma's.

When I had my 2018 Tacoma, I found that it could be avoided by simply accelerating much more slowly. But my thought was that you shouldn't be forced to accelerate slowly in order to avoid the feeling of a loss in power (early shift).

 

Can someone explain to me what the "flex-lock up" or torque converter is?

And why would they stop it from happening when in S4?

 

Definitely an all around different feel when driving in S4. It feels like YOU are (or can be) in control of shifting based on speed/throttle input. Its predictable.

But in D you can definitely feel that it's mapped for economy. I'm not complaining. It's very similar to my 2nd gen Tacoma's.