Toast fennel seeds, peppercorns, and coriander in a dry saucepan over medium heat for 30–60 seconds until fragrant.
Add vinegar, water, sugar, and salt. Stir gently while heating until the sugar and salt dissolve, then pour the brine into an ice bath to cool.
Place cucumbers, garlic, and dill into four sandwich bags, and add just enough cooled brine to cover them. Seal each bag using the water displacement method.
Put the sealed bags into the ISI whip, fill it with water up to the max fill line, screw on the head, load one nitrous oxide canister, and charge. Shake gently, then refrigerate for 2 minutes.
Slowly release the pressure to avoid forcing brine out of the cucumbers, and let the mixture sit for 5 minutes.
Charge the ISI whip with a second canister, refrigerate for an additional 2 minutes, and slowly vent the pressure again.
Remove the bags, strain out the brine, and serve the pickles. They’re best enjoyed within 2 hours for a satisfying crunch.