How can you cruise only half the Panama Canal?

Cruising down the Panama Canal has to be on the bucket list of every serious cruiser out there, normaly from Fort Lauderdale to L.A or vice versa.
For people who either don't want to or just don't have the time most cruise lines also offer the option of only cruising half the canal so you can say you can experience it without it taking up a good chucnk of your holiday.
But here's a question for you. In a great big cruise ship, how do you cruise only half a canal, it's not like you can turn around half way through?
Is it?