How to use masks in scid for opening training

Question

I've been trying to understand how the "mask" feature works in scid and how to use it for training opening lines. So far I did the following:

1. open a database of master games in scid
2. open the tree window
3. while playing through an opening line, right click with the mouse on a move in the "tree window" and select add to mask for all moves that interest me
4. in the same "tree window" right click on a move and select a marker such as "main line"/"dubious"/"Include". This step I only did very rarely.
5. save the mask by left clicking "tree window"->"mask"->"save".

I did this whole procedure separately for my white and black repertoire, so I get two masks: "whitemask"/"blackmask".

Am I on the right track with this procedure? What else could/should I do with the mask?

How do I use the mask? I noticed that sometimes in the chessboard window, it shows me the possible alternative moves from the mask by arrows. However I did not figure out when it does this, or how to enable this feature (which I find quite useful). I'd like to play through an opening making moves for both sides, while having the arrows show me the possible moves according to my mask. How can I do this?