The Positioner is the ingame tool, or to be more clear, set of commands on numpad that allows to mess with currently active weapon holster node (Holster only and not specific weapons) - move and rotate.
These are the commands:

num . = on/off

num + = add operation
num - = subtract operation

num / = reset all
num 0 = show current state

num 1 = choose operation move
num 2 = choose opearation rotate

num 7 = choose dimension x (heading)
num 8 = choose dimension y (attitude)
num 9 = choose dimension z (bank)

num * = mark changes for current save

So, you choose the needed operation (move or rotate) and dimension(x,y,z) and increase/decrease values with num + and num -

It works in realtime so you see all of the changes instantly.

num / resets all of the changes to default values and num 0 will show a message box that will display current active dimension and operation. Pressing num * will mark the changes and they will persist when you save game. all the changes you save with num * are saved in f4se co save.

Keep in mind the tool doesn’t restrict you with anything, meaning that you can have weapon holster, like 50 meters from character, but at the same time the holster will always stay attached on its initial parent bone

If you have fo4 hotkeys mod installed and unable to use positioner keys, just use them by holdig ctrl or alt, for example alt + num 0

Changing Keys
1.06 version introduces ability to change default keyboard keys for this tool, they can be found in ClassicHolsteredWeapons.ini under [PositionerKeys] section and changed to any other key. 
Here you can find keyboard key names and their codes

Positioner tool is a player exclusive feature, its not going to work for npcs.