Initially, the combo buttons are undefined, and look like this:
If you press and hold a combo button, like you would a radio preset, until you hear a "blip" (if you have sound effects enabled), and when you release, the button will have as its definition the current history, and it will be a bit darker:
If that button previously had a definition, and if you have confirmation enabled, you will be asked to confirm that you want to change the definition.
To erase a definition, start with an empty history (immediately after a Shake, Home, or Restart) and do the same thing. If you have confirmation enabled, you will be asked to confirm that you want to erase the definition.
For instance, let's look at the combination Right+Swap+Left+Swap, or "RSLS" in the history. Start with Home, touch Right and Swap and Left and Swap, and then touch and hold the "A" button, like this.
Now touch Undo four times to get back to the home position, and then touch "A" to get to the same position.
Any sequence of moves, if repeated enough, will get us back to the position from which the sequence was started. The number of times that takes is called the order of the move. How many times will we have to execute "A" before we get back? This is what we see touching "A" a second time.
When we touch "A" a third time, we see this, the home position, which tells us that the order of "A" is 3.
As we watch closely the animation of the numbers moving as a result of the "A" move, we see that the 12 numbers move in 4 groups of three, which explains why the order of "A" is 3.