This effect is available in the "Game rules" tab.
First of all print your comment and click on the "Set price for user to read this comment" link:
Note: This is a "paid" effect which means that you’ll have to spend a certain amount of your points to see the full text of the comment.To apply this effect, select the text that you want to be visible to other commenters for free and click the "Select" button:
The selected text will be displayed to all users for free and the other part of the comment text will be automatically hidden.
Then set the amount of points other users should spend to see the full text of the comment and click the "Post" button:
After the effect has been applied, your comment will look the following way:
Note: The "Pay to open" effect is not available to the commenters, for whom Expert mode is enabled.