Sax80:

The 'tap' function button allows the user to override {set} the tempo in play in Amplitude 3. Click on the button 4 or more times in succession at the desired tempo, and Amplitude resets at that tempo. The current and adjusted tempos are displayed in the box immediately to the left of the tap button.

Clarification and detailed explanation are on pp. 60-61 of the PDF Amplitude 3 User Manual.