Try this website. Look at the diagram. 

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Cell surface receptors have three domains, an extra-cellular domain, a membrane domain, and a cytoplasmic domain. These receptors have an extra-cellular ligand that is a messenger. When this ligand binds to the extra-cellular domain of the receptor, it causes a change in the physical characteristics of the receptor. 

The part of the receptor that acts as a tyrosine kinase is inside of the membrane, in the cytoplasmic domain. With Kit, when stem cell factor binds to the extra-cellular domain, the receptor re-arranges its physical conformation, and then the cytoplasmic domain becomes competent to do tyrosine kinase activity. Part of this re-arrangement of its physical characteristics includes auto-phosphorylating itself in a region near the active site which binds the substrates. 

In a normal situation, the cytoplasmic domain doesn't become competent for tyrosine kinase activity if stem cell factor does not bind to the extracellular domain and stimulate the physical re-arrangements. However, in GIST, mutations have occurred in kit. As a result, the cytoplasmic domain constantly acts as a tyrosine kinase even when there is no message from stem cell factor. 

Sti-571 does not bind where stem cell factor binds. Instead it binds in the area of kit that act as a tyrosine kinase, and blocks off the ability of kit to auto-phosphorylate. Its like sticking a cork in a wine bottle. Sti binds to kit in the cytoplasmic domain, not the extracellular domain. 

An over-expression of kit would mean more copies of the kit molecule. I am not sure one way or the other if this occurs. However, what does occur is "consitutively" activated kit. Meaning that kit performs kinase activity even when it shouldn't. So instead of having XX number of kit molecules doing kinase activity for YY percent of the time, we have XX number of kit molecules doing kinase activity 100% of the time. Its doing kinase activity even without signals from stem cell factor. 