If you are marking well you should feel the effort in your quads. This allows you to get your hands low and also gives you much better balance. This also helps your balance. Obviously though, if you are trying to prevent a high throw, that arm will have to be up. In general, the arm on the force side should always be as low as possible, since it is much harder to throw a good throw from higher up.
Many throwers make weak fakes without ever intending to throw.
If you can pick these up, you will not be sucked in to following them and leaving an easy throw open. It is not your job to block an open side throw. That is the defenders' responsibility. If you let the thrower break because you tried too hard to block an open side throw, that is your fault. Loudly When the Thrower Throws This allows the defenders to glance around to see where the disc is, since they will otherwise be watching the receivers.