segwit nested in p2sh
There was a longer discussion thread in bitcointalk, I proide two references:

https://bitcointalk.org/index.php?topic=2333913.msg23758008#msg23758008

https://bitcointalk.org/index.php?topic=1682183.msg21389041#msg21389041

(Both threads are worthwhile to read in it's entirety). 
In short: before nodes talk to each other, they verify the capabilities. And a segwit node would reduce the information for a non-segwit node, so that he can understand the transaction details.