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This is not a very good practice I'll tell you, hshshs, but they have. What you need to do, after registering the domain, make a page is fully optimized for the same keyword field. Optimize the title, headings, content, images, etc.. After the page to start earning position, maybe you have to win some links too. Only after she earns a good position, you wait a few weeks and makes the 301 redirect. Your site takes the position that page for that keyword. In the experiments I did, most sites have not lost the position. But some werelost over time, but still worth it. If only register the domain and do the redirection at the same time, will not haveany advantage and the possibility of the search engines interpret that is amanipulation is greater. I hope it helped