Clicker under accelerator pedal?
 

Why is there a little button under the gas pedal? I was hoping it was the rocket booster, but it wasn't. I am sure it serves some purpose, is there a difference between flooring it all the way vs pushing til it clicks?

I noticed the clicker forces the downshift. On some ocasions, having the pedal all the way down doesn't downshift the transmission, and only pushing the clicker downshifts.
But, RPMs shoot to 4500, and it being a diesel, just dissappoints in that RPM range.

I don't know if that's the indended purpose of that button.

The button is to kickdown the transmission. I always try not to push it because I like steady accel not crazy downshifts.

I guess if you need to really get-off fast you need to click it, because it feels like X5 is always taking off on the 2nd or 3rd gear. Doesn't let you feel like sinking into your seat, head-into-headrest speeds. :)

If I just press the pedal (and click) on takeoff, I experience this 1-second horrible lag, where the vehicle just sits there, before it takes off.
The trick is to hold the break pedal -> push accelerator halfway -> release the break pedal while pushing the accelerator all the way.
In this case, you do experience the head-into-headrest acceleration.

Because of the 1-second lag, I have to be careful when pulling out of my lane, with vehicles coming down the road behind.

I thought the acceleration did not function (disabled electronically) when you had a foot on the pedal, both to avoid stuck pedal disasters and to avoid excessive wear and tear.
Did you try it with the shifter in DS and transmission dropped to 1?

Acceleration disabled with foot on break pedal?
In normal (D) mode, and it works fine.

Bingo, the SAVs transmission shifts down when you press the pedal to the kick down and in theory provides the maximum acceleration.

ECS Tuning

My experience on the clicker is....

If I dab it quickly (& immediately release), then quickly press it again to accelarate, I notice the transmission downshifts immediately. It seems more responsive.

But if I simply press the pedal and keep my foot down to accelarate, it'll have usual the downshift lag experienced by many.

Dabbing and releasing then pressing the accelarator pedal to accelarate seems to produce quicker response than being in "DS" (or Sports) mode.

I generally do this if I quickly want to overtake and/or quickly leave a side road on a busy highway.

Again the steps are: Quickly dab (all the way down) & immediately release, then press pedal to accelarate as per normal.

Try it and tell me if you experience the same thing I do.