In simple terms, it's the same as "Speed Hold" on earlier types, with some refinements.
It will maintain the selected Airspeed or Mach Number, set the appropriate Climb Thrust if the selected Level is above you, set Idle thrust if the selected Level is below you, and level off at the selected level upon reaching.
There's not a lot of difference to VNAV during the Climb, both functions are trying to achieve pretty much the same thing, with some small differences. During the Descent however, VNAV PATH will adjust thrust
and speed to seek the desired flight path. FLCH during descent, however, only maintains the pilot selected speed and Idle Thrust, irrespective of the Flight Path.
I don't know about reaching the selected altitude in two minutes
Hand Solo! Does that mean that if I select FLCH at 1000 feet after Takeoff with a set Altitude target of 35000 feet, that it will climb at 17,000 feet per minute!
Perhaps you meant that it will achieve the selected speed within 2 minutes? That sounds more reasonable.
Regards,
Old Smokey