I've never actually seen Andy or Jez play rack harmonica, and I must say I assumed their recordings were done as overdubs. Both of them play really well, but I don't play with a rack because I feel a bit limited, compared to what I can do with the gob-iron in my hands. But expressive playing also comes from altering the shapes inside the mouth and through breath control. I was really thinking about blues playing at the time, and Duster Bennett could get great sound from his harmonica in the rack (he was also playing guitar, high-hat cymbal and kick drum at the same time!). Have a listen
But his best playing was hand held.

The simple line on crossharp (playing blues style on a harmonica in a different key) is that it is a bit easier to bend draw (suck) notes than blow notes, so playing this way transfers some of the more important notes from blow to draw, to give that characteristic bluesharp sound.