Some details about the R rear retractable spoiler :

The rising rear spoiler is borrowed from the Cabriolet for a little extra attack angle, but the development team found that a Gurney flap would create too much stress for the parts, so a diffuser under the engine was employed — and it is said to be absolutely functional. The rear-wheel steering is critical in the 911R's ability to go without a large fixed rear wing — thanks to the added stability it brings.

https://www.pca.org/news/2016-03-01/12-things-you-didnt-know-about-911r