If you miss that chance, you may have trouble until the loads comes off the sail as the boat comes head to wind. The safest technique, therefore, it to keep the fall of the sheet ahead of your feet, flick the sheet to uncleat whilst still out on the wire and then release the sheet completely on to the on to the double bottom forward of your feet as or before you come in off the trapeze. Sheet In Pick up the sheet again as you cross the boat to start pulling the genoa in on the new tack. Get the sail most of the way in as the boat comes round on to the course and then get out on the trapeze as the load comes on. These upside down cleats do mean that it is very easy for the sheet to recleat itself during a tack if you hold the sheet in you hand as you come in off the wire.
Safest thing is to uncleat, let go of the sheet, and then come in.
They do not allow the sheets to run out quickly and smoothly enough during the tacks.