…continue example…
Your HUD needs to add the interface:
Your HUD event graph could look like this:
This would draw a very basic HUD that gets the current health and energy values from your jetpack blueprint every tick and draws both values in the top left corner of the screen. (Right on top of each other but the principle should be clear 
)
For this application your don’t need the interface output pins. These would be needed if your HUD would do something with the values, for example checking whether you clicked on a HUD element to refill those values, and than return those modified values to the jetpack blueprint to update the values as well.
(Note: if your interface returns something, the event node won’t show up. Instead a function is added to your HUD as soon as you add the interface blueprint.)
Why you need to pass those values by reference I can’t answer you. I did not experience this problem before.