Stepping off and back on while moving at an obstacle. The movement is identical at any speed — so drill it slowly until it's automatic, and only then add pace.
This is where cross beginners burn the most energy — not from a lack of strength but because the movement isn't automated. The sequence: brake in good time, hands on the hoods, unclip the dominant foot and swing it round behind you past the rear wheel while pushing the bike slightly away. Your feet land one after the other and you go straight into running with no pause. Getting back on, open your hips towards the bike, push it until the saddle is beside your hip, jump so your thigh meets the saddle and slide across rather than crashing down. Typical errors are braking too late and coming to a standstill after landing. What often goes unnoticed: the choice between hopping and dismounting is made well before the barrier — clear track and firm ground favour the bunny hop, mud or a crowd favour stepping off.