Your top hand should start out straight enough to enter with a vertical paddle, but during the stroke your top elbow should bend as you pull, so that your forearm ends up contacting your chest and your top hand comes down toward your hip.
Having a loose grip allows the paddle to find its own path on the exit. If we have a death grip we might be lifting water up. Entries always slice in from the side and we don't want to stir up the water as we're putting it in. The Exit is the paddle feathering and finding the path of least resistance. The Catch is the part of the stroke after your entry. Make sure we get off as clean as possible, as little wasted energy as possible. The most important part of the stroke is the beginning of the stroke; it sets up the power through the whole stroke.
This video shows how our entries come mainly by rotating inward. This slices the blade into the water, cleanly and deeply, without wasting our shoulder strength. For all our strokes, we rotate in then down for power. In the Thrust, our bodies move forward first, then we rotate in to enter. Power starts with legs driving our bodies back, then rotation and shoulder strength are added for more power.