The placement of Sprite-Units allows units to have <default Starcraft HP max>/<custom set HP max> hitpoints.
To place sprite-units, go to the Sprite Layer of Starforge, and make sure the button above the unit tree says "Units" not "Sprites".
For example, if Nexuses are set to have 200 HP in the Unit Editor, a sprite-unit nexus will have 750/200 HP.
Don't believe any rumor that different values in the HP2 box can do this, any nonzero value for 'HP2' increments the max health by one.
changeing the hp to be above the max will cause units to look different (this would be the purpose in many cases), but if you do certain amounts I think it can make a unit invincable -certain amounts on certain units mess with other properties.