Okay kiddo, let me explain what extension transference means in a way that you can understand. Have you ever used a pencil with an eraser on top? When you write with the pencil, you use the tip to make marks on the paper. But if you make a mistake and need to erase it, you flip the pencil over and use the eraser to remove the mark.
Now let's say you have a different pencil without an eraser, but you still need to erase something. You might try to use the end of the pencil, where there's no lead, to do the erasing. This is called extension transference, because you're using the extension of the pencil (the end that's not meant for writing) to try and accomplish a different task.
Extension transference can happen in lots of different situations. For example, let's say you're used to using a computer with a mouse to move the cursor around on the screen. If you switch to a computer with a touchpad instead of a mouse, your brain might try to use the touchpad like a mouse, even though it works differently. That's another example of extension transference.
So basically, extension transference is when your brain tries to use one part of an object for a task that it wasn't designed for. It happens when we're used to doing things a certain way and have to adjust to a new situation.