Minimalist apps are great, but sometimes this app takes minimalism too far and becomes downright confusing.
Places to click:
Hamburger button in the top-right corner — Settings. Note that the hamburger button disappears whenever in the middle of a period. To get it back, long-press in the center of the circle.
Short-press in the center of the circle — Pause/resume.
Long-press in the center of the circle — End the current period and immediately move to the next period.
Click on the "Pomodoro" / "Short break" / "Long break" text near the bottom — End the current period and immediately move to the next period. (becomes unclickable when in the middle of a period)
Click on "reset" at the bottom — End the current period and immediately move to the next period. (only visible when in the middle of a period and paused)