I think it is part personal preference and indeed a change of grip depending on the sword use would be very fast...However is it not ultimately dependent on the size of the sword hand compared to the hilt? The chap shown has huge hands and he could not possibly hold the small Tulvar grip . Using the sword with the finger over the guard makes for a very powerful down strike...or maybe he is trying to get one finger cut off so that his hand will fit??...
