Foil slings are, in short, just a sheet of aluminum foil, folded up until it becomes sturdy. They are used to enable pressure cooker users to lower pots, bowls, cake pans, or whatever heatproof vessel fits in their pressure cooker, down into their pressure cooker. The sling allows you to easily lower the vessel down into the pot, and then easily retrieve it at the end of the cooking cycle.
Here's how to make a foil sling:
Start by tearing off a 12 inch x 12 inch piece of aluminum foil.
Fold it in half.
Fold it in half again!
Fold it in half one more time! Now it's super sturdy and can support the weight of whatever you'll be lowering down into your pressure cooker.
Now, place the sling underneath whatever heatproof cooking vessel you'll be lowering down into the pressure cooker. Pictured is my Pressure Cooker Frittata.
At this point, you just grab the arms of the sling and use them to lower the vessel down into the pot, where it will most likely sit on top of the trivet. Some recipes will call for you to tent foil over whatever you're cooking so that condensation doesn't drip down into it and make it soggy.
I typically use my foil slings for multiple uses if they don't get food all over them.
Happy pressure cooking!