After looking it up in FIG, I have another question: what's the difference between a Markelov (D) and a vault catch (B)? From the drawings it looks like the only difference is height -- is this correct? Is it just left to the judge's discretion whether to credit the skill as a vault catch or a markelov?
The Markelov is a 1/2 twist in the air...the rear vault is only a 1/4 twist. In other words, you can hold on longer when doing the rear vault. It shouldn't matter if it is done out of a giant as that is a separate skill.
Also, the Markelov is only a "C" for MAG.
Read the descriptions below very carefully for the twist info.
Set a spring board up in front of a low bar, on the other side of the low bar have a stack of mats up to about one foot below the bar. Have gymnast run and jump from spring board, block off the bar in a hecht motion and perform a half turn to feet on the stack of mats. The drill can then obviously be tweaked so the entire Markelov can be achieved from the jump over the bar and onto the stomach on the stacked mats...best drill I have seen to perform this relatively easy trick.