The masking is based off of the active area. The position of the active area is defined from the lengths of the blanking areas. The blanking areas consist of frontporch, sync and backporch. The sync should normally be left alone, the frontporch is calculated from sync and backporch. Hence backporch is the value that you need to adjust to shift the image over (this goes for both horizontal and vertical position).
To quote the wiki (my bolding)
Horizontal/Vertical backporch length
Sets output backporch lengths, which by default are set to match expected input backporch lengths. Effectively adjusts image position.