>>2842119 AG and LC typically work for the entire duration of the motion they need to be activated for. An example of a motion that depends on low center is SUNMI - Heartburn by Lan_Ruo, as it's a motion capture (?) with leg IKs disabled, so without it the model slides around. Arm-adjusting morphs are also meant to be activated once, but I usually need to perform limb corrections (anger-vein.svg, sob.svg) anyway. In the case of HL, I prefer to correct bone keyframes instead.
On that note, MikuMikuMoving has a bunch of useful plugins, including the somewhat fickle ApplyOffset (if it breaks and starts moving random bones along with your selection, just reload the project; it also does not work in the same way as motion layers, so you can't just rotate a bone the way you want using motion layers and then copy the rotation to get the same result) and ApplyRotation, which rotates selected bones AND THEIR OFFSETS using an arbitrarily positioned pivot, which can be useful if you have some sort of a horizontal reciprocating motion for the purpose of procreation or mere worldly pleasure that you want to make vertical, for example. ApplyOffset would only change the orientation and position of the center bone keys alone, so the motion (offsets) would still be horizontal. MikuMikuMoving plugins are searchable by MMM用プラグィン and English terms, and there's also this large list of links for various MMD-related software:
https://note.com/fow77/n/n3bb62d0c64d9#WU9tN 