keterys
First Post
From Martial Power:
Ankle Cutter
Heroic Tier
Prerequisite: Halfling, rogue, Sneak Attack class feature
Benefit: When you hit a Large or larger enemy and would deal Sneak Attack damage against that target, you can forgo rolling Sneak Attack damage and instead cause the target to be slowed until the end of your next turn. Using this option counts as using Sneak Attack for the round.
Currently beguiling curse does not have that last line which is necessary to make it once per round.
The presence of text does not imply a general ruling. It is entirely possible, for instance, that if you removed your bolded line that Ankle Cutter would continue to work exactly the same way. It would just potentially be less clear.
For example, the avenger article gives avengers an opportunity attack when an enemy moves adjacent and explicitly says it doesn't apply when forced movement or teleporting, but the general rules for forced movement and teleport _already prevent opportunity actions_. The Storm Shield does lightning and thunder damage when someone hits it, and notes only those with resistance to both resist that, even though that also is the general rule. Etc.