I had another play with this technique over the weekend, and I think I finally cracked it. You have to lay the yarn in and then carefully crank until it is captured by the closing latch, before you lay the next colour in. See slightly out of focus shot below, it was the best my little Nikon coolpix could manage:
The multi-colour yarn is trapped, and the background (which I left threaded up) is laid in for the next few stitches. On my Imperia, there are two half-circular cut outs on the base of the yarn carrier, and I discovered that as the needle gets to the rightmost corner of the right cutout, that is when the latch closes and traps the yarn. If you try doing this in the centre of the yarn carrier, or before the feeding hole in the carrier, it very often drops a stitch and it's difficult to keep your eyes on both yarns. If you always ensure the last yarn is trapped then you can lay in the new yarn and forget about the old one. Try to keep one yarn above the other (as in handknitted colour work) so that the yarns don't get twisted. The navy is the upper yarn in this case, as I left it threaded through the mast.
And the finished sample, not a dropped stitch in sight (but then the first attempt was awful and mysteriously got filed in the circular file). :)
I did wonder whether using holding position would work better for this technique but I think it would probably result in two-colour tuck instead. Something to think about.
Edit: because apparently left and right are interchangeable in my pea brain! =)