See if this helps.
I recently talked to a math specialist and she explained it like this.
On the overhead draw a number line with positive and negative numbers. Then make a character (I liked to cut out a person I printed off the computer and put on an oiverhead so the students can see which way he is facing).
So, take the problem given 7 - (-10)
Have the character start at 7. Since, the next number is a negative number, have your character facing the negative side of the line (to the left)
Okay... so explain to the students that every time you add a number, your character takes a step forward and everytime you subtract, he steps backward. Therefore, since it is subtraction, the character then steps 10 steps backwards landing on #17.
Let me try another problem. -4 -(-6)
The character starts on -4; then, since the next number is a negative, the character faces to the left. Finally, since you are subtracting, the character takes 6 steps backwards landing on 2.
I did this with a fifth grade class and the students really understood. I also had the children act this procedure out and they really got it. I hope this helps and doesn;t confuse you. Let me know if you have any questions.