Pulse Sensor Working Bottom-Side-Up Better?
From what I see, this is a pulse sensor that work with light. It emits a bright light beam, that is reflected by the pulsing blood. To work, it must send the light directed to your veins. It should work well on your finger, because the probability to target a vein directly is much greater with a small finger then e.g. a wide wrist. Also the veins in your finger are closer to your skin and the oxygen situation is more different while pulsing, so the blood has more impact on the reflection. 

The second curve might be a light source in your room, or an electrical wave. (?) I doubt it is your heart, because, if the numbers below the graph are milliseconds, then your heartbeat would be between 10-15 beats / second, i.e. 600 - 900 per minute. That would be too much even for a hummingbird ;-).