When you want to slow down, just "ease" your sheeting angle slowly.
Depending on your rig size this could be very little, or as much as 15 degrees.
If you "ease" the pressure in your rig, it will most certainly result in a decrease
The whole "point" here is that you do this slowly.....
DO NOT "sheet out" suddenly as this will upset the balance of the board, the rig and the fin and may result in some sort of "crash".
As far as controlling the rig when you are hooked in, if you have your harness lines positioned/balanced correctly, you will be able to take both hands off the boom momentarily and the rig won't do anything, just give you the same amount of power you had when you took your hands off.
If your rig increases or decreases it's power, or tries to fall forward or to the rear, when you remove your hands momentarily, then your lines are not "balanced" and your rig has the ability to jerk you around all over the place.
If your rig is powering up more, that usually means you have your lines too far back on the boom and 2 things are happening:
#1 as the speed and power increase, the rig sheets in a bit more (because the lines are too far back and it's "pivoting" around the front line attachment point) and it also wants to fal forward which increases it's power and gives it more fore/aft leverage over the resistance (you the sailor).
You can set your harness lines so the rig actually "eases off" when it gets overpowered.
Move the front line until the rig neither falls forward or to the rear, then position the rear line attachment point so the rig tends to "ease out" if you stop pulling in with your back hand.
This makes the rig very much like a car.
If you lift off the accelerator pedal, a spring pulls the throttle closed and your cars slows down.
Getting your lines balanced so the rig "eases off" when you release the pull with your back hand makes your "throttle" close slowly, just like in your car.
Unfortunately, you can't "experience" how all this works without fully committing to the harness lines.
Once you get it right, and learn to trust that you can ease off if required, handling gusts (with modern twist off at the top sails) is quite easy.
The sails are designed so that the pressure/power in the top of the sail (the upper part of the sail has the most "leverage" over the sailor) automatically reduces in gusts by the simple expedient of having the leech "open up" and spill some wind, easing the overall pressure in the rig.
Hope this helps,
Hope this helps,