Add the cream cheese, sugar, allspice, flour and salt to large bowl and use an electric mixer to combine.
Then one at a time, add the eggs, whisking until combined.
Add eggnog and vanilla extract and mix by hand with a spatula until combined.
Pour the batter on top of the crust inside the springform pan. Tap the pan gently on a table or kitchen counter to remove any air bubbles. Cover the pan with aluminum foil.
Pour 1 cup of water into the bottom of your Instant Pot insert, then place a trivet inside. Set the springform pan on top of the trivet.
Put the lid on the Instant Pot, turn the valve to the sealing position, and set to High Pressure / Manual Pressure for 50 minutes.
Once done, allow for a 20 minute NPR (natural pressure release), then perform a quick release to remove any remaining pressure.
Take the cheesecake out of the Instant Pot and remove the aluminum foil. If needed, gently blot any exesss moisture off the top of the cheesecake with a paper towel.
Cool the cheesecake on a wire rack for 1 hour. Then cover with plastic wrap and refrigerate for at least 6 hours before adding the topping and serving.