Both techniques are valid and worthwhile, but it really depends on your goals. If you want to tone and sculpt your biceps, I recommend the free-standing curls. This will force your smaller support muscles to get involved, which tends to give a better overall shape, as well as help your balance, stability, and control when using those muscles. The downside is that you cannot lift as much weight this way, because your less-developed support muscles must take a percentage of the load, and won't be as strong. The result is that you will gain tone and a little strength, but you aren't going to rip your biceps and gain any significant size. If you want to build and strengthen your biceps, I recommend the braced curls. My technique is to brace my elbow against the inside of my thigh, so that my triceps are resting on the top of my thigh. This will protect the elbow, give some friction so that your leg is also helping to keep your arm up, and still allows a full range of motion. This technique will give your weaker support muscles a rest, and allow your major group to receive the focus. This should cause your biceps to gain the desired strength and size. The downside is that your support muscles will get no training, and while you will gain your strength and size, you will have poorer fine motor control and are less likely to get a balanced, sculpted look. In either case, using a full range of motion and a moderate pace is critical. Stretching your muscles will help prevent injury, as well as give the entire muscle the benefit of the workout. Moving too fast will cause the weights to move because of momentum, which doesn't help you. My routine is to do the braced curls on biceps day. After doing a few other exercises, I'll to the curls in descending sets until failure. This makes sure that I get a good muscle-building and strengthening session. On all other days (except the day after biceps), I'll do a few sets of free-standing curls with moderate weight, to give me the shape, balance, support, and tone I'm looking for. Hope this helps!

i sit on a chair and put my elbow on my knee then lift. making sure my muscles get fully extended

HERE you go. definately use something with a preacher curl machine affect. chair whatever. this will train only your biceps, giving you a WAY BETTER ARMS than just curling standing up.your biceps will endup round and perfect. good luck!