what is the "turn key once to start" feature called?
 

i'm very used to e46 where you hold the key in position III to start. when i got my e53 4.8is in 2019 it caught me off guard when i turned the key once and had to let go and it still cranked on its own.

i was curious how this works or what its called? i would love to research the wiring for it more due to my curiosity.

its almost like a push button start but in key form

from the 2001 owner's manual:

BMW X5 4.4i:
Your BMW is equipped with the convenience
starting feature. Simply turn the
key to position 3 (starter) and then
release it immediately.
The starter continues to operate automatically.
The automatic starting mode will not
operate if the battery voltage is low.

Automatic start. A separate relay on the starter activation line. Can only be implemented with certain engine control unit versions.

https://bmwteka.com/wds/en/e53/139b5a53

Auto start is what most manufacturers call it. My '08 Nitro had it. Goes back to an '04 Benz C240 too. It's been around for a while. If you hold the key, the ECM on most cars will cut fuel to keep it from starting.