short burst of effort to get some momentum and then ease off the pressure / torque while you change.
Trying to change when under full load isn’t good for the drivetrain; it’s the only time I’ve bust chains / bent teeth on the cassette.
On a more zen level – select a gear you can spin comfortably to the top of a climb when you’re at the bottom, and then live with it. If it’s too easy, just pedal faster, but if you have to change, changing up is easier because you’ll only be changing up when you have extra capacity in the legs to be able to spin up some speed and then softpedal. If you are in too hard a gear and need to change down, finding the power to spin up some speed is already less likely.
Or get a SS and don’t change gear at all – just use your three SS gears; sit down / stand up / get off and push.