Use the arrow keys or WASD to control the character. Think of the character as a plane banking (thanks OpusFreiling). For most of the time, you should be parallel to the ground (without banking more than 45 degrees). To get hearts that are surrounded by black ones, you should be perpendicular. The controls may be difficult to get used to but are needed to maneuver to certain hearts. Try to collect all 100 red hearts without hitting the black ones (it is possible, I did it). There are different outcomes depending on how you land, where you land, and how many hearts you have. Be sure to try different things out!