Normally, you'd specify the VPN traffic via access-list, such as:
access-list vpn permit ip 172.16.0.0 255.255.0.0 172.17.0.0 255.255.0.0
Then your crypto-map would have something like this:
crypto-map myvpn 10 match address vpn
Between them, traffic which is originates at 172.16.0.0/16 and is destined to 172.17.0.0/16 will be sent through the VPN. Any other traffic will be handled normally by the Pix.
If your access list was "permit ip any any", then all outbound traffic would go through the VPN. With the Pix this is a problem, because the site would have no internet access unless there's a proxy server or some other way to the internet from the remote side.
The short version- split tunnel is good for site-site VPNs. You activate it by specifying only the interesting traffic in the ACL.