I have hill assist on my (DCSB V6 MT) Sport 4x4,
The truck knows when you are on an incline (not sure of the incline degree range of engagement).
At the end of my road is a hill with a stop sign. It works every time I hold the brake hard enough to hold the truck on the hill.
It holds for about 2 seconds then fully releases. I actually think it holds as hard as you hold the brake when you depressed it.
So if you hold light on brake equals the hill assist holding the same pressure,... hold the brake harder then so will the assist.
Basically it keeps the same pressure you initially placed on the pedal. BUT in order to engage it MUST be on a incline.
I find it helpful on a smooth clutch release and never a downside to it,..... 2 seconds is not an issue.
I have tried to release the clutch and go immediately on the hill, and it does seem to release immediately as well.
It should not be on "TRD OFF ROAD" with a manual transmission only on an automatic transmission as per Toyota...... (I have it on my TRD Sport 4x4 with a MT)
Strangely it is ONLY on automatics in every trim other than TRD Sport with a MT. (unless Toyota screwed up and installed mine on a MT)
Source: https://youtu.be/beog9J5DBEQ