If a buffer named *Trace* exists, the rewrite mechanism will record some useful information there as it operates. The original formula is written there, as is the result of each successful rewrite, and the final result of the rewriting. All phase changes are also noted.
Calc always appends to *Trace*. You must empty this buffer yourself periodically if it is in danger of growing unwieldy.
Note that the rewriting mechanism is substantially slower when the *Trace* buffer exists, even if the buffer is not visible on the screen. Once you are done, you will probably want to kill this buffer (with C-x k *Trace* RET). If you leave it in existence and forget about it, all your future rewrite commands will be needlessly slow.