Toyota got lazy when they switched to the 5 speed tranny in '05. Instead of modifying the shifter/steering column for the extra gear, they just put a button on the dash. Its the same as if there was one more position for your shifter for 1st gear.
It took me a while to figure that one out as well, My friend who is a Toyota mechanic didnt even know what it was for. Shift to second and press it. A little L should appear next to the 2 on the gear indicator. Its basicaly first gear. Works well along with 4x4 low.