Originally Posted by Bichon

The A/C compressor in the IS is a variable displacement design. It uses some clever engineering that increases or decreases the piston's stroke on-the-fly. By changing the displacement, it is able to vary both cooling capacity and engine load from full capacity to near zero. Under heavy engine load such as WOT, the computer simply reduces the displacement of the compressor to reduce parasitic load on the engine -- no need to switch it off.

Incidentally, the variable displacement design is why you don't hear the compressor cycle on and off, and why leaving it on has a minimal effect on fuel mileage during mild weather.

Originally Posted by Bichon

You are mistaken. The IS250 does have direct injection. It has a 4GR-FSE engine, where the "SE" suffix means direct injection. (And a forum search for "ticking noise" will turn up dozens of posts from IS250 owners concerned about the sound of their injectors)