As soon as I am below close hauled, I lift the centerboard enough to stop it gybing. Then I play with it.... a little more down for power, a little more up if the boat feels "stuck to the water" or in breeze, overpowered. The more borne off I am, the more I pull up the CB. I even pull up the CB all the way to sail by the lee down to a mark, in light air flat water (if you are trying this, let the main all the way out, ease the vang way off, and pull the pole back to rotate the spinnaker away from the main).
In light air we sit forward, perhaps not as far forward as we might while going upwind, but close. As speed through the water increases we move aft somewhat. At low speeds we seek to reduce the hulls wetted area by sitting forward and getting the flat stern out of the water while immersing the more rounded - and lower wetted area - of the bow. As speed increases, wave making becomes more important, and we slide aft just a little to lengthen the waterline and let the boat sail on her lines.
Close reaching in very light air, and beam or broad reaching in light or medium, we often sit astride the centerboard trunk - motorcycle fashion. The crew sits about as far forward as he can, and I sit forward of the mainsheet jammer up against the crew. This keeps our weight in the center of the boat and forward. It also improves the feel, and allows us to balance the helm - or even turn the boat if necessary - by simply leaning slightly to one side or the other.
Provided the spinnaker pole is back slightly, the crew can see the luff of the spinnaker quite well, and the skipper can see far better than if they were sitting on the leeward side.
The tension on the sheets is telling you how much pulling force you are getting. If the tension is very low on the spinnaker sheet and mainsheet, there is very little force pushing the boat. Often the first signs of a slight puff will be a little heel, a little bit of corresponding weather helm and a a slightly increased load on the spinnaker sheet. You can simply lean to windward, get the boatspeed up, and then bear off a little to ride with the puff.
Sailing downwind in light air is really a series of trade offs. You can sail higher and generate more speed through the water, which brings your relative wind forward and increases it, but at the price of sailing a longer course to the mark. By sensing the feel of the boat, you can sail it as low as you can, without losing flow over the sails. The tradeoffs are often quite clear if you compare your speed and angle to boats around you. Even on a reach, the rhumb line may not be the fastest course, you might be able to go noticeably faster by sailing a few degrees higher, and then come down low - going more slowly - but only for a short time to get down to the mark.
"Getting light, come up a bit"
is typical as the crew tells the skipper that the boat needs to be headed up for a little more speed and relative wind. Similarly
Is an invitation to bear off and use the speed and or puff to get more downwind to the mark.