Secondary Stats Secondary stats increase when one or more primary actions are performed (except for throwing). Secondary stats are: Time Units Health Strength Energy/Stamina Secondary skill points work quite differently from primary skills: It does not matter how many primary actions are taken; all that matters is that you did one primary action (land a non-IN attack on an enemy, react to something, not panic when Morale is below 50, or use a psi attack) -- your secondaries will increase all that they're gonna. It also doesn't matter if you did more than one kind of primary action.
From https://www.ufopaedia.org/index.php?title=Experience