When you are caught while at the service line or even closer to the net and know you will have to handle a very hard short hop, you must use NO backward or forward motion of the racquet head because there isn't time! Also, the racquet head motion induces more chance for mistiming. Simply block the ball back.
As such low bouncing balls rise to contact your racquet face, the mirror effect dictates that the balls will leave your string bed on the rise. With practice you will be able to adjust your racquet for the placements you want to make.
When the ball arrives on your forehand side, stay in balance while turning or hopping sideways as you position your left foot forward. Lean way over -- those with young enough knees should drop down with the right knee close to or even touching the court surface. Extend the racquet sideways with the head perpendicular to and touching the court surface and in line to intercept the ball. Be sure to hold it with a solid grip. With a friends' help feeding you short hop balls (not easy), you must practice and you will soon get the "feel" of how much if any to lay the face back to clear the net with your return.
Then, whether the ball contacts the court very close to your racquet face, or many inches in front of it, you must be sure you are ready to jerk the head straight up very fast -- coinciding with the ball striking on the court! Some balls that strike the court even further in front of your racquet will rise with a high bounce and your racquet head will also need to rise high enough to meet them!
You will be AMAZED at how many balls you will catch right on your sweet spot, even on the somewhat higher bounces, and with practice you will be able to then add some drive to your "catch and return" efforts.
If you need to return on the backhand side. it is best to utilize the two-handed grips that are not generally recommended when close to the net. Two handed strength is needed for these short hops!
With practice, you will be able to place these shots very well by angling the racquet head a little forward or backward.
If a ball is driven at your feet so hard that you have no time to squat down, you should just thrust your racquet head down to block it and pray a little!
Remember, anytime you are able to get a ball back over the net, you afford your opponent the opportunity to make an error, especially if they are "out of position" because they didn't expect the ball to come back!