RE: How to avoid "spin outs"?
Spin out is pretty common, it happens to all windsurfers, from time to time.
Sometimes it's caused by a bad fin.
Sometimes, even with a good fin, it happens, and techniques that reduce the loading on the fin can help.
Sometimes just tuning your rig a little better can make all the difference.
I agree with o2bnme that what board you are sailing, what fin you are using, and what rigs, windspeeds and surface conditions can all be factors here, it's rarely purely a technique issue, unless you tend to "jump on" your fins way to early.
The foils on windsurfing fins REQUIRE a certain minimum flow of water over the foil before you can really "push" on the fin.
Push at too low a speed, and the fin stalls and spins out.
This is why most of us head a little off the wind and try to get fully planing before we put much pressure on our fins.
So, give us a little more information here, and I'm sure we can give you some suggestions to solve your "spin out" issues.
When you put "pressure" on your fin, what is the rail to rail attitude (the roll angle) of your board? Are you sailing with the upwind rail lower than the downwind rail?
Are you trying to sail "on the fin" with the lee rail slightly (very slightly) lower than the upwind rail?
Are you trying to push the tail of the board down, toward the water, or trying to push the tail away from your body?
Either of the above techniques works, but one works best on very small boards with usually a single rear footstrap, where the other works much better on a wide board with a large fin.
At no time will you be able to "pull up" with your back foot (unless you are chop hopping or jumping) and thus reduce the pressure on your fin.
Shifting your weight to the front foot can also cause problems.
Give us a little more info to work with please.
Hope this helps,