Whatever you do don't let your DIN be too low, you should take this seriously and experiment with it yourself until you get it right. If it's too low whenever you land switch you will always just pop out and eat total shit even though you did everything perfectly. I know because I was doing that constantly, and one time it resulted in torn cartelige in my knee and put me out for the season. Just experiment with it yourself to find the right setting, make sure you don't set it to high because you won't pop out and your leg might break or you might fuck your knees.
Also, it doesnt take someone working at a ski shop to set your DIN. All you do is get a screw driver(flathead or phillips depending on your binding), theres a little knob on the back of your binding, and a little knob on the front of your binding to raise or lower your DIN. The knob on the back controls when you are to be released out of your heels, (like up or down motion making you fall). The knob in the front of your binding controls when you will be released out of your toes which is like sideways or twisting movements that cause you to fall. If it is too low then, try to figure out if your pre-releasing out of your toes or pre-releasing out of your heels, and adjust your bindings accordingly, don't do it too much at once though, just like .5 at a time like if yours is at 5, then go up to 5.5, if its still too low go to six. etc. etc. Make sure though if your raising it you do both skis.
Also if the guys at the shop are telling you to set it a 4, (now i don't know because you weigh 120), but i would think thats still a bit low. I weigh 160 and mine are set at 9 and that's about right, I pop out when I need too, not just by simply landing switch. Set them at what you had your bindings at last year, and raise it up if you have too. People who are riding park and racing have to have their DIN higher than normal because of the kind of stuff we're doing.
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