Do NOT automatically make ALL torrents download sequentially. This is really detrimental to the network. Only for media which you want to start playing ASAP, so really only manual for most cases.
Where? All you had to do is click 'Help' to end up here https://support.tixati.com/sequential_downloading
By default it will wait for 3 seeders to become sequential which is a wise choice to not kill off the very torrent you're downloading. Don't tell us we didn't warn you ;)