This isn't any different than Curse of the Dark Dream or Crown of Madness, which also have no duration.
To me, it's clear the target becomes eligible to make saves, after the power has been sustained (since there is nothing to save against before that). I prefer it this way because it's more in line with ongoing damage powers, where you are guaranteed to have the effect apply at least once. So while I like the Warmachine approach of "Effect: Target is nibbled (save ends)," I don't think that works as intended in this case. The intention is that the target shouldn't be able to make a save until the power has been sustained.
For any power that has Sustain, but no Effect, it's as though it had the line:
Effect: This power may be sustained.
And then the description within Sustain X should be sufficient to resolve it.
If the power is meant to be sustained only on a hit, then I would expect it to be indented underneath the Hit section (eg. Hurl through Hell).
Maybe there is something I'm missing about the problem, but it all seems clear to me.