99.9% of the time, don't worry about the CLB/CON switch. I suspect many pilots on type are not sure when it would be used(perhaps myself included) as it really never is used. But the button is there and allows you to immediately select the climb thrust setting that you entered into the FMC(the three options being CLB2, CLB1, or CLB), instead of having to make several button pushes in the FMC to achieve the same thing(or waiting for the FMC to automatically set it as it was programmed to do by the pilot prior to takeoff).
Maybe during the takeoff at a high power setting, climb thrust is set at 3000' AAE in the FMC but for whatever reason decide you want the climb thrust setting set in the FMC earlier, press the button and thrust changes to what you programmed into the FMC on the ground.
Maybe you set up the FMC prior to departure so that the climb thrust is greater than the takeoff thrust due to derates and ATM and for example, a selection of full climb thrust(CLB). You decide soon after becoming airborne that you want more power from the engines right away as you can see an aircraft on the TCAS ahead of you at the same altitude. Push the button and the power comes up. Or maybe the setup in the FMC is the more typical reduction in thrust at the thrust reduction altitude and for some reason you want a reduction right away, perhaps for a new low altitude level off. Press the button and the thrust reduces, maybe a bit or maybe a significant amount depending on the takeoff thrust versus climb thrust difference.
The aircraft is smart and knows that one engine is inop. You have lost an engine at low altitude and you have lots of derate/ATM for the takeoff and want Max Cont right away. Push the button. Or maybe you are at a very high thrust setting and you want the good engine to reduce to Max Cont. Push the button. Or patiently wait for the proper speed for it to automatically happen(as is normally done), or make the appropriate selections in the FMC.
Or maybe you are doing pattern work for touch and go's and you want a reduction in thrust soon after takeoff to climb thrust as you will be leveling off soon. Perhaps at 600 feet, you push the button.
As for your question...it is automatic at Vref+78 and you confirm on the EICAS screen by noting that TO(or TO1 or TO2) indication has changed to CON and you might see the thrust levers move and engine indications change. Or you can manually do it in the FMC or you can press the CLB/CON switch both of which are not normally done.
Last edited by JammedStab; 25th Apr 2017 at 14:37.