When using PerfectIt, you can switch back and forth between Word and PerfectIt as you check. However, you cannot edit documents while PerfectIt is scanning for errors. When it is done scanning, you can jump back and forth between the document and PerfectIt (exactly like a spell checker).
In general, we recommend that any changes made while working with PerfectIt should be smaller changes (editing individual words, phrases or paragraphs). If you decide to alter the document more significantly (e.g. by adding pages of text or large tables or graphics), you should restart PerfectIt so that it can gather information on the latest version of the document.