Question about Power Leech

So according to Guildwiki, the skill description is as follows:

If target foe is casting a Spell, that Spell is interrupted and for 10 seconds whenever that foe casts a Spell, you steal up to 1...6 Energy from that foe.

This skill got me very intrigued when I saw a monk using it in TA, does it really work? comes to mind. In this case is the energy stealing portion of the spell conditional upon you interrupting a cast in the first place? Or does the hex gets applied regardless?

Thanks expert mesmers

This is a two part spell. The first is the interrupting part. The second is unconditional not dependant on interrupting a targets spell.

Edit: Of course this is just an upfront judging by the text. Don't take my word as being absolute on this matter.

Try it out with a guildie.

You need to interrupt a spell for the rest of the hex to do its job. So if you don't interrupt then you've just wasted a spell.