Takes a little practice and daring ..... on a lightish day just let the main out and keep bearing off til you are by the lee .. get your tell tails flowing backwards from the leach to the luff ...... heeling the boom up a bit ....... the key is getting airflow over the sail backwards from what is "normal" ..... its fast!
Basically you sheet out with the boom around the 90 degree mark and then bear away until you see the flow reverse over the sail. This can be spotted using your tell tales. If the leech starts to flick be very careful as you are right on the point of gybing.
You should practice on a light-ish day (but not too light) so you can see the effect of the wind.
Be prepared to get wet though. Once you have it sorted it is a very useful weapon to have when sailing downwind especially in mixed fleets.
usually when you're sailing on a run the tell tales will be fluttering, if you head up you'll be on a broad reach, head down and you'll be sailing "by the lee" ie reverse flow on the sail,
to get by the lee flow, keep the boom out 90 degrees in light-medium conditions, head down, raise the centerboard so the top edge is around 4-6 inches off the deck, don't raise it too much or you'll slip sideways
as you head down you'll see the flow on the tell tales reversed and you'll feel speed/power as if you were on a reach,
while you're learning, stay alert and low in case of an accidental gybe.
If you keep sailing deeper until the sail gets back winded then you will get a good idea how far you can go by the lee,
if you're about to gybe you can either go through with the gybe or head up to stay on the same tack.
Had this situation at the weekend at our annual Laser open event. The boat goes so far then will not go any further. All you can do is ride out the gust, gybe as soon as possible and then come back to the right line at a better angle. Much faster than forcing the issue and going for a swim.
am just learning to sail by the lee, by using a GPS to display speed at various sail positions in a steady 10knots breeze.
Using the sail square as a baseline; 8km/h with full shape and vang/kicker eased.
Sheeting in until windward tell-tales flows: boat slows, until:
tell-tales on leewide side of sail flow; 10km/h.
I found I can sail at the same angle to the wind, gybe and let boom go far forwards; full length of 12.5m mainsheet out, and then get tell-tales flowing on an extreme broad reach. 10km/h.
In light steady winds, I could achieve the extreme reach more easily with less loss of speed in the process of doing it.
However, in gusty 20knots the extreme reach drives the mast to windwards and the swerving and work done to stop death-rolls is inefficient and costs speed.
This image shows computer-generated theoretical flows over a sail:
If you imagine this is sailing by the lee on starboard tack, there would be flow going past only the aft/starboard side of the wind indicator, causing anti-clockwise spin.
Have a good set of telltales (especially have a telltale up in the top third of the sail) and a wind-indicator so you can really tell where the wind is from. Go out on a light/medium wind (8 knots ish) with steady wind that's not shifty. Bear away onto a dead run and get settled, make sure you have on foot hooked under the toe strap and locked against the inside of the cockpit underneath you. This means you can move onto other side quickly if you need to. Have the sail a little past 90 degrees out and the vang soft with quite a lot of twist, but not loads. Gently bear away by the lee and be prepared to shift your weight to windward if needed. The sail is very hard to gybe once you have good laminar flow over both sides of the sail. If the boat roles to windward the bear further by the lee slightly, let the sheet out a touch and at the same time push off the inside of the cockpit and semi stand up towards the other gunnel quickly.
Hope this helps.