The Hill Start Assist is on all Toyota/Lexus Hybrids. When stopped on a hill a conventional IC can be made to maintain postion on the uphill stop by "stalling" the automatic transmission torque converter with a light press of the accelerator. Heats up the trans and wastes fuel but we've all done it!
Since the Hybrids are very often not running the IC at all while stopped, they have included the Brake Pedal actuated HSA that simply holds the main brakes for up to three (3) seconds displaying the yellow icon and sounding a chirp. You can override this at any time before 3 seconds by pressing on the gas. Otherwise it just times out, chirps again and you may begin rolling backwards. It does not engage no matter how hard/fast you hit the hooks if the car is still moving forward, must be already stopped first.
Because the RX450h has the HSA (and the detailed Fuel Economy Graphs in the Nav Screen
) the RX350 gas version has them also. As mentioned earlier, the LS460 system is more Limo Chauffeur oriented, engage the Hold button for a standby setting sort of like Cruise Control then press a separate button to hold.