This stoke is used to go sideways on the move and is very useful for obstacle avoidance, rafting up etc. It’s strictly a sea kayaking/white water stroke and quite an advanced one.
To perform this stroke you need to be paddling at a fair speed. Place the blade out to one side and angle the face of the blade at an angle of about 45 degrees facing the bow. The top arm stays static in front of your forehead. As the stroke tends to turn your boat, you will need to edge the boat towards the active paddle side (blade in the water side). This will counteract the turning movement of the boat. I cover edging in another later tip, but it’s essentially leaning the kayak but keeping your body as upright as possible.
If the boat starts to turn rather than move sideways, adjust the position of the blade in the water forwards or backwards or apply more edge (lean more towards the blade in the water).
Common mistakes include incorrect placement of the blade in the water causing the kayak to turn and poor positioning of the top arm. Raising the top hand too high or straightening your bottom arm can put your shoulder at risk.
Here’s a good video demonstrating the technique.
https://www.youtube.com/watch?v=Kg1wbQq4WRw