Be aware that your book is talking about IAE engines re the ignition. CFMs don't come on with flap.
Re APPR IDLE there is a certificated minimum time that the engine must achieve TOGA power during an approach. I think it's 6 seconds.
From THR IDLE (min power) which is desirable on descent to TOGA would be greater than six seconds, so they need to bump the idle up a bit when landing.