Get an Attribute's Value before it changes?

How do I get an attribute's value as another attribute before it changes? This seems
like the hardest thing to do when coding, and I want to learn how before I can clean
up my StencylForge items and improve them.

Set the recording attribute to the changing attribute before  you change  it.

For Example

If Damage =/= 0

Set Recorded HP to HP
Set HP to HP-Damage
Set Damage  to 0