A "Pulse" is the term used by WhatPulse to define the action of transferring your statistics from the WhatPulse client to the WhatPulse servers. The WhatPulse client, which is running on your computer, has to communicate with the servers to tell them how many statistics (keys, clicks, uptime, network traffic) it has counted.
WhatPulse uses this technique to prevent unnecessary communications to the servers and to preserve your bandwidth. The pulsing frequency is controlled by the user, which means you! You can manually pulse by clicking the 'Pulse' button on the 'Overview' tab, or use the 'Pulse Now' in the tray icon menu. You can also configure the client to automatically pulse whenever you'd like.
To set your client to automatically pulse, go into the 'Settings' tab and choose the 'Automatic Pulsing' sub-tab. From there, you can choose whether to auto pulse on the reach of a certain statistics, or at a specific time in the day or week. A common setting is to configure the client to pulse every 15,000 keystrokes.