Every pixel stores a palette slot, not a color, so editing a color in an unlocked palette recolors the whole sprite at once, and switching presets maps each pixel to its nearest new color. The onion skin shows the previous frame faintly under the one you're drawing, which keeps movement smooth between frames. Switch the preview to Tiled to see the sprite repeated 3×3, which shows whether a tile's edges line up.
The right mouse button erases. Right-click a palette color to draw with it on the right button instead. The Image menu flips, rotates and outlines the current frame, or every frame with Apply to all frames turned on.
Shortcuts: P pencil, E eraser, G fill, I pick a color, L line, R rectangle, V move, M mirror, Shift+H and Shift+V to flip, comma and period for the previous and next frame, and ⌘/Ctrl+Z to undo. The sprite lives in the page's link, so Share › Copy link hands someone the whole animation.