Prepare the Pan and Instant Pot: Grease a 6-cup Bundt cake pan. Place the trivet in the Instant Pot inner pot and add 1 1/2 cups of water. 

Make the Batter: Combine the brown sugar, butter, egg, and milk in a bowl with an electric mixer; beat until smooth.
Add the baking soda, vanilla extract, flour, cinnamon, salt, nutmeg, and allspice; mix on low speed until well blended. Fold in the diced pears. 

Fill the Cake Pan: Spread the batter in the prepared cake pan and cover it loosely with foil. 

Place the Pan in the Pot:  Place the cake pan on a foil sling or baking sling and lower it onto the trivet. 

Pressure-Cook: Secure the Instant Pot lid and turn the steam release valve to the sealing position (this is automatic on many newer models). 
Choose the manual or pressure cook button (high pressure) and set the timer to 45 minutes. When the time is up, let the pressure come down naturally for 10 minutes and then move the valve to the venting position. 

Invert the Cake: Transfer the cake to a rack and remove the foil. Let the cake stand for 5 minutes, then carefully invert it onto a plate and drizzle with syrup, if desired. 

Serve and Enjoy: Slice the cake and serve it on dessert plates. Enjoy!