Adding to the above...on the 787, FLCH behaves almost exactly the same as VNAV SPD. While speed is on the elevator, it sets the thrust to achieve the desired altitude change in approximately two minutes. So a 3000 foot FLCH climb or descent will result in about 1500 fpm up or down. Naturally, when there is a very large difference between the present altitude and the MCP altitude, this results in climb thrust or idle thrust. On descents, the autothrottle still goes to hold mode once it is set, as with the older Boeings.