people hold the mast in a jibe?

don't know anyone who does that.
The only time I used to hold the mast sometimes was tacking in really light conditions as I found i could flick across to the new side of the sail faster doing that, but with a bit of practise I go boom to boom all the time now.
Back on topic thou, interestingly i've heard two different methods of going boom to boom during the rig flip. I think guy cribb promotes reaching over to maintain mast foot pressure. matt pritchard promotes reaching under so you maintain your body position in that low aggressive bent knee position, thou he did say that the other way can work too. Both are well respected windsurfing coaches, I honestly think both ways work fine.
I actually do a bit of both, grabbing under and over, and sometimes grabbing under with an underhand grip on the new side (OH MY GOSH).
To be honest I think doing the boom shaka all the way to the front of the boom is the most important step to smooth rig transition. And do it all the way, your pinkie should be hard up against the boom clamp. Some people do it a bit half assed and don't really grab right at the front of the boom and it shows in sloppy rig flips.