There are two modes, GESTURE mode and BUTTON/JOYCON mode. Press Spacebar to switch between then.
In GESTURE mode:
Use one finger to rotate the heart, use two fingers to slice into it to reveal the insides.
In BUTTON mode:
If you’re using a keyboard, press and hold V to rotate the heart, press and hold B to slice into it to reveal the insides. If you’re using a JoyCon, press and hold any face button to rotate the heart, press and hold the trigger to reveal the insides.
You can adjust how quickly the heart spins by pressing the + and – keys.