open your bail, keep your thumb on your line to avoid backlash, and give it free spool until the board pops back up.
ways to prevent that from happening is use 9' rods or longer, as the higher your rod tip, the less chance of it happening.
on my boat, I have my anglers stand on top of my casting deck, which gives another 30" or so of height.
when the board "dives" your likely going to loose the fish, as you experienced. so open your bail and let it pop up. if the fish is still on, thank the fish gawds.
that is the reason i don't use normal inlines for copper...once the fish hits, after the weight of the copper, the boards go under immediatly...loosing your big fish.
hope this helps.