Yeah, when it was used the first time it seemed to not work particularly well against strong-willed people - otherwise, they would have just turned Erza into a card in the first place and called it a day. One of the trapped people was able to cut Lucy's ropes from inside his card, for example, and he was just a normal guy (IIRC).
My assumption is that, depending on the strength of your will, you can resist or eventually escape from the card on your own. Elfman's will was, by the point that Cana zapped him, basically nothing, so he got zonked out and stayed in the card, and everyone else trusted Cana and so stayed in the cards willingly.
And also I think that was a pretty cool moment for Cana so I don't even care how it worked. ;)