The Turtle/Rabbit slider adjusts the response of the 4 stick (flight control) channels. You can check it in the channel settings. Down in the bottom center of the screen is the 'Final output monitor'. This shows the current value of whatever channel is currently selected on the right hand side of the screen. Thr, Ail, Ele, Rud are the first 4 listed under the 'Channel-Function(Output)' section. Default is channel 1 (Thr). It should read 50% since the throttle is in the middle. If you now move it full up, it should read 100% (in Rabbit mode, slider all the up/away from you). If you continue to hold the throttle full up and move the slider down to Turtle mode, you should see it move back to 80%. If you move the throttle full down, you'll see that the end point is 20% (Turtle mode) and 0% (Rabbit mode). So 0 - 100% in Rabbit mode and 20 - 80% in Turtle mode.
Here are my results for the channels:
Turtle mode
Thr 20% <> 80%
Ail -60% <> +60%
Ele -60% <> +60%
Rud -60% <> +60%
Rabbit mode
Thr 0% <> 100%
Ail -100% <> +100%
Ele -100% <> +100%
Rud -100% <> +100%
So basically 60% of range in Turtle mode vs 100% of range in Rabbit mode. And adjustable anywhere in-between. If you're familiar with the concept of dual rates, then this is basically the same with adjustability thrown in.
I haven't found a way to adjust how much the Turtle/Rabbit slider adjusts, but you can edit the endpoints of the 4 channels to increase/decrease the overall range of those channels individually.
Warning! The following involves changing the channel settings, so if you're not comfortable doing that, don't do it! Seriously, you can really mess things up. If you are OK with it, make sure you write down or take a picture of the default settings, since the reset button doesn't reset to factory settings, but instead resets to zero.
For instance, I wanted more yaw authority, so I increased the Rudder channel endpoints to 150. Now when I move the Turtle/Rabbit slider, I get 90% - 150% instead of 60% - 100%. Still 40% change from Turtle to Rabbit mode, but just more of it.
Before you say it, yes, I'm aware that 150 - 90 = 60, but 40% of 150
is 60