Hey @ard! Welcome to the forum!
Hum... it seems your understanding is correct. What you've described should indeed move the servo continuously in one direction. Here's an code example that does exactly that. https://code.quirkbot.com/program/#!/57bb5eb8d98853497a2cb10e
Are you sure you are using a continuous servo (I mean the physical hardware)?
Do you have the Developer Kit that comes with two servos (one continuos and one normal), or the standard Robotic Creatures Kit, that comes with only one servo? The standard does not contain a continuous servo...