When placing the Pet On Wheels under an airline seatplace the wheels in first at an angle and face teh wheels toward the aisle. If the seat is a little too low for the carrier to be put under enough to stisfy the flight attendants, push the tab in that allows the carrier to collpase, as if you were going to collapse it to store it, but it will only collpase enough to go under the seat. It will crush your animal.
Putting your feet up on the frame of the carrier while it is under the seat is a great position for your back and makes your travel much easier on you and your pet will feel much more secure knowing you are close by.