I am very glad to meet you. 

How do those two mods you listed work exactly?

How do those two mods you listed work exactly?

Hello!

hkxPoser allows you to modify poses as much as you want; basically you can create a totally new pose. It runs as a separate program and has no preview system or anything, so it's a bit cumbersome to use if you want to get a pose just right: you have go to the game to see what it looks like. 

NiOverride Pose Adjustments is an in-game tool to make small adjustments to poses. It takes some time to get the hang of it but once you do, it's very handy and useful; you immediately see the result. Making major adjustments is also possible but very, very difficult to get right. At the moment this tool works only on the player character, but hopefully Sesamin will later add the functionality to adjust also NPCs.

Both tools can also be used to modify animations.

In practice it's best to make the larger adjustments with the hkxPoser and then fine tune the result with the NiOverride Pose Adjustments.