Here's how I understand it:
The whole weeks goes on normally. The HOH nominates two people, veto is played and the power can be used or not. We get all the way to Thursday's live eviction. Before the votes happen, Jessica can stand up and halt the entire eviction. No one votes. No one goes home. The next HOH competition is played, as usual, and a new HOH is crowned. Basically, the week will have been a waste for the HOH whose eviction was halted. They will get no one evicted.
Jessica has three or four (can't remember which) weeks to use the Halting Hex, so this week counted as the first. If she and/or Cody are on the block in the next few weeks, she can halt the entire eviction. It covers both nominees, and there are no replacement nominees.
I think Jessica really made a mistake by A) telling Kevin that she had the power and B) announcing to the whole house that she has it. I agree with a previous poster who said Paul should go ahead and nominate Cody and Jessica this week. Even if the veto is used, one will remain on the block. Let Jessica use the hex to stop the eviction. Then move on the next week. If they refrain from nominating them until the hex runs out, they will risk evicting people who can be used as votes against them later. Paul might consider it a waste of his HOH, but making them use the hex would be a win for his side. He could keep his numbers and Jessica and Cody would not be safe the following week.
