How do amphibians use positive pressure to draw air into their lungs?

When the frog opens its mouth, Air flows in because it's in the nature of gases to flow from high pressure to low pressure ( Air to mouth ). When the air is inside the mouth of the frog, it closes its nose trills ( to avoid the escape if any air trapped ) and closes its mouth. When nose and mouth are closed, the air is trapped inside the mouth. Then, the frog raises its jaw which presses the air inside the mouth towards the lungs, because again, air under higher pressure (in mouth) goes to a place where there's lower pressure ( the lungs ). This is positive pressure inhalation.

Then O2 is released from the air to the lungs, and CO2 is picked up. The frog then lowers its jaw, to lower the pressure in the mouth, to let the air travel from the lungs, to the mouth and into the outside,

hope that helped!