I'm an intermediate level snowboarder, riding blues and blacks. I think i'm decent at linking turns both skidded and carving, I can also dynamic ride well to control speed.
The problem for me comes when i want to go straight! Sometimes i either want to bomb straight down the mountain, or even just cruise at a good clip at the bottom of a run as it levels off. I feel really unstable going straight. I'm not trying to ride flat so i keep a little toe or heal pressure, but that seems to turn me too much and so i'm zigzagging more than i want. I see other guys fly by me just bombing or cruising straight down. It looks like they're riding flat, are they?
If i ride flat it feels like my board wants to spin/pivot/rotate on me. I'm goofy and it feel like my back/left leg wants to move backward rotating me counterclockwise. So I consciously use force to keep my rear leg stuck behind me and it uses a strange groin muscle. hah doesn't seem right. It almost feel like if i rotated my shoulders forward it would help counter this rotation but that doesn't seem right.
Things i'm keeping in mind is to put as much weight as i can on my front leg and to keep my shoulders parallel with the board.
Stance is symmetrically ducked. It almost feels like i want my rear foot more ducked so that it pulls the back of the board in but i don't think thats the proper solution.
I'm riding my new T Rice Pro which is a hybrid rocker and it likes to pivot a bit easier than my old tradition cambered board. So now i'm really focused on this problem.
You gotta keep even pressure on both legs. If your back legs is sliding out then you're off balanced. I had the same problem before. I can't go too fast but it's a decent speed. There was a thread on this made a week or two ago. Unfortunately with me having the flu, I have a hard enough typing let alone searching :/
Generally be on an edge even a slight one when maching, sounds like you are twisted up or slightly in the back seat. get your shoulders, torso and hips parallel to the board and quiet and ride centered or slightly on the nose...squirrelly board = backseat...shift hips forward/sideways toward the nose...1/2- 1 inch.
also check and make sure your foot/binding is centered waist wise across the board.
I used to have this problem too. You just gotta have your body properly aligned (parallel) and stacked over your board. Don't do unnecessary movements, one technique my friend thought me is imagine that you're an arrow and just point downhill. If you do that you will be able to flat base and go straight. Another thing to picture is that if you just let a board go downhill it will go straight and will not catch an edge. Make sure to keep your knees and legs loose so you can absorb bumps.
i know i have a problem of doing unnecessary carving..one time on the blue i let myself get on edge slightly and just hold it while trying to keep myself balanced and shoulders parallel to the board...i acutally didnt turn much and was able to catch speed and catch up to my friend. sometimes when i am on edge i have the tendency to carve just by habit and because i like carving to control my speed...but this time i just let myself relax, look ahead and stay on edge without trying to carve a lot....the less movement the better i think.
I find that having a beer in each hand helps keep my weight distributed evenly. Of course if you take a drink out of either one you must match the displacement by consuming the same volume from the opposite canister. It get's pretty technical, so I don't recommend it unless you have a degree in hydro physics. You might want to just stay centered and loose, the good thing is speed wobbles are almost impossible on a snowboard.
I remember the frustration of not being able to flat-base when I was starting out. Loose & balanced is the name of the game. You need quick reaction time and that just comes with practice & developing good muscle memory. Leo's preferred lesson will not help you unless the terrain is also really smooth you need to be able to react very quickly to bumps, ruts, etc., that will want to redirect you and throw you off that balance resulting in the dreaded edge-catch-to-scorpion.
Also remember you can pressure an edge without being on an edge.