Stein,

With a spinnaker boat, it's a balance between keeping your speed up and allowing yourself the crew enough time to manuever the sails through the gybe (non-spin boats are a little different in this regard). If you slow too much during a gybe, you create a large difference in your boat speed and the wind speed and the pressure on the sails (and flipping the boat) increases. If you can keep a good deal of speed, the difference between the speed of your boat and the wind is less and you stand less chance of capsize. However, gybe too fast and you or your crew looses their footing or looses control of the sails. Practice allows you to gybe more quickly and more safely in heavy air.