If you wish to pause the current timer, you can do so easily by clicking the play/pause button (>) from the main window of activeCollab Timer
Another way to pause or start an active timer is to right click the menu bar icon or tray icon. If you prefer to use keyboard shortcuts you can also use the space bar to start and pause the current timer.
The activeCollab Timer is also capable of detecting idle time and will be paused automatically if you are away from your computer. When you get back, the timer will start again automatically. If you want to change this default setup though you can do so via the Preference menu.