The hypotenuse is actually a shared side of the two triangles. (The line segment that crosses the middle of the rectangle) The hypotenuse is the side of a triangle opposite the 90 degree angle. We can use this for hypotenuse congruency. The diagram also shows the shorter legs of the two triangles are congruent. (The legs refer to the sides that make the 90 degree angle)
So we have hypotenuse congruency and leg congruency.
