To proof for an obvious and prolonged nose hold: call "Alert!" then move in and pay Jackie at source. Once your dog has learned a solid nose hold, no longer use a marker word "Yes!" but instead just call "Alert!" and pay directly at source.
Proofing Jackie's nose holds while alerting three hides in a container search:
If Jackie breaks her nose hold to look up at the handler when I call "Alert!", simply freeze, wait for Jackie to return to her nose hold, then go in and pay at source. Jackie is paid for holding her nose at source, even while she hears the alert being called.
The tin can game proofs for distractions. Start by putting odor such as lysol or windex on a paper napkin inside a couple tins, and the source odor in another tin. Pay Jackie at source when she nose holds at the correct odor tin. After you pay at source, pick up the odor tin, toss a reset cookie, and replace the tin in a different spot to continue playing.
Move with Jackie around the tin can game, but don't get too close or crowd her. The handler can allow her to search independently while still supporting her by rotating on an imaginary person hole cover while staying in one place.
In this clip we see Jackie starting off very well playing the tin can game, and then she hones in on the food scents (especially the chicken pot pie!). Jackie then displays what I call the classic border collie out-of-body experience: they overthink so hard that they stare off into the distance until they recover to the task at hand.
Gradually increase the game's difficulty by adding more tins with food inside -- leftovers from your own dinner for example. Jackie particularly loved the tins with chicken pot pie and a pretzel with peanut butter on it!
The decision to play on or off lead depends on the search space. In a large, open area where the search space is not securely contained with fences or walls, for example, the handler will probably choose to search on lead to maximize time by keeping your dog on lead within the search area.
Searching off lead tends to increase Jackie's independence and search speed, and is a nice option when searching areas that are large but contained such as a school kitchen or gymnasium, or areas that are small but obviously demarcated such as during the tin can game. When searching off lead, stay within fifteen feet of Jackie with your hands in position as though there is an imaginary lead between dog and handler.
