Photo credit: New U.S. Navy photo by Sonar Technician (Surface) 1st Class Ronald Dejarnett
"Breaking the sound barrier (Almost)."
The SPD/MACH push button toggles how airspeed is commanded on the flight instrument – either Indicated Airspeed (IAS) or Mach number. IAS represents the speed of air molecules moving through the airplane with reference to sea level. It is a representation of the air plane's kinetic energy level. On the other hand, the Mach number measures an aircraft's speed in relation to the speed of sound at a certain level.
As altitude increases, the air becomes colder and less dense, and the speed of sound decreases due to fewer air molecules and less vibration within air molecules to propagate sound waves. Therefore, as an airplane climbs higher, its Mach number increases even if it flies at the same IAS. Approaching Mach 1.0 (100% of the speed of sound), shockwaves start to form at some areas of the airplane, causing severe drag and vibration, this is also known as the Sound Barrier.
Since airplanes normally maintain a relatively constant IAS at high altitudes, Mach number becomes a more critical factor, thus a more preferable speed command reference.