segwit nested in p2sh I understand that P2SH addresses are used to allow "segwit unaware" wallets to create outputs spendable by segwit wallets (please correct me if I got something wrong). For new clients I think I got it: they will check that the script cointaned in the witness matches the script hash and execute it.

What I do not understand is how this  output is spendable by a segwit aware client to the eyes of an old client. Because it clearly doesn't look like an anyone can spend. How does it work?

Thanks.
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.