I went out today at Canton on my 78ltre Tabou pocket wave in overpowered conditions c25-20kts + and 6" chop. I haven't sailed it a lot and I was using an 18cm weedie.
I know you can't have any real backfoot pressure when you sail it normally or it spins out but today was weird. I was really overpowered & hanging on for grim death and we were flying along but going diagonally sideways..not straight sideways like a normal spinout and I was still doing 23kts +..
. I suspected I needed to pull in with the back foot and then put more weight on the front leg but I didn't want to for fear of getting thrown around the front as I was so overpowered..?? Would that have fixed it? I did straighten it at times and it didn't happen all the time. It was survival sailing so I wasn't sailing as ' neatly ' as usual..
The weed was thick but I didn't really notice it with the small fin ( except for the occasional big clump)..
Fins to small, I run 17cm weedies in my twin...So that's 2 of them to your 1. Sounds like your sail was too big as well if you were cooked in 25knots, You were overpowered because you just had the wrong setup for the conditions. If your struggling with your kit it means your not commiting, Your sailing with self preservation in mind. Commit, bear away and get traction (gets a little scary) then pinch up wind to slow your self down. Your sailing diagonally because your fighting your kit, Get the right size fin and sail for the conditions and everything will be sweet....Even on 40knot days 
Sailing sideways, usually indicates weed on the fin, even though it's a weedy, at low speed it can still collect there. If weed is the cause pulling your back foot in won't fix it, but jumping the board might, (not something to try if you're out of control!)
Bad sideways drift can also be caused by the fin partially stalling, doesn't happen that often normally it goes completely, especially with a fin that small. But can have similar causes, as jh says fin not big enough, not enough downhaul, too much backhand pressure and too much back foot pressure.
At Lake George earlier this year, taking off in shallow water and thick weed, this was happening to me a lot, I had to look for deeper clearer water to clear the fin and start again.
Thanks..In usually would have put a 3.2m on in those conditions although its an older 90's twitchy thing and needs consistent 30kts +. I could have been using more rear hand pressure. I usually use the harness fully and don't use my arms at all but I was ready to try & unhook at any time yesterday.
Could have been weed on the fin too..it was bad. One time I waterstarted and realised I couldn't see my hook to hook in as I had a 6" thick belt of weed around my waist... Had to stop and clear it. Everytime you started you had weed trailing off your feet and boom etc..Hope the wind has cleared it out..
That board is my only US box so until recently I only had the 18cm fin . It'sa makani freemove? I think and I got it for those big winter days over 30kts..I did get a 23cm one late last year as well but I haven't used it yet and I wouldn't have wanted it yesterday as the board was scary enough in the chop with the 18cm!
Hopefully today will be more normal for a sail ( provided I can get some b sleep)..
The freemove is more a freestyle fin which is designed to slip easily, if your not use to this sort of fin you will sail diagonally. This sort of fin requires you to use the boards rail to get grip more so then the fin. I use the 20cm Mano weed in my 80lt and it's sweet, Never gives way. 
Select to expand quotejh2703 said..
The freemove is more a freestyle fin which is designed to slip easily, if your not use to this sort of fin you will sail diagonally. This sort of fin requires you to use the boards rail to get grip more so then the fin. I use the 20cm Mano weed in my 80lt and it's sweet, Never gives way.
Durr.. That explains a lot..
