The throttle based 1st and 2nd lockup is only noticeable if you have a heavy enough load on a hill. Without enough load, you won't stay in 1st or 2nd long enough to notice it because there is a time delay of about 30 seconds once the revs are high enough. It will lock up as low as 15 MPH in 1st. It's really noticeable because the revs drop so much. Drops from something like 2800 down to 1900.

It is useful for controlling heat on steep, curvy roads. Rather than let is slip at a constant 1/2 to 3/4 throttle, you can get on it and make the TC lock up, then back off to check speed if necessary as you come up on corners.