Jammed
I'm going to do this pretty top level, let me know if it's not clear (as if you wouldn't
)
Lock and Lapse is pretty much standard on all modern FADEC - Pratt started it first on the initial PW4000, GE adopted it on the GE90-115B (GE also uses it on the GEnx on the 787 and 747-8).
What lock and lapse does is 'lock' the takeoff power setting, then 'lapse' the N1/EPR power set with altitude/airspeed in a prescribed manner. As a result, say you initiate takeoff from sea level, lock and lapse will determine the N1 (or EPR) that you'll see at 2k pressure altitude - which may not be the same N1 that you'd get at that throttle setting if you'd initiated the takeoff from 2k pressure altitude.
While not necessarily obvious if you've not worked it, lock and lapse makes the power setting and limits analysis much easier to define and analyze across the takeoff envelope. It's particularly helpful for analyzing assumed temp derated takeoffs where the throttle set is well below full rated takeoff - without lock and lapse you need to go through all the various altitude/airspeed power setting tables to make sure you get at least as much thrust as you would at the assumed temperature - with lock and lapse the analysis becomes trivial...
Does that help?