I use the key. I did it by mistake one day. You have to turn it really hard before it works! After the first time it gets easier. I think they used a standard ignition switch and put a permanent stop on the rotating motion. Well, not-so-permanent!

 

I thought I used my index finger...but no! i went to lunch and found myself thumbing....The horror!

 

(my post above is a joke... please don't try!)

 

i used my thumb on my friends Rx7

 

so many people blabbled, but forgot to answer what finger they use...
index or pointer... don't know what it's called, but the one to the left of my thumb!
(on the left hand of course lol)