If you're a newly registered user, you need to raise your level in Yahoo Answers to 2 to get dofollow backlinks. After that, getting dofollow backlinks from Yahoo Answers is pretty easy. You just need to put your links in your Answers and on your profile; all links will be appended with rel=”dofollow” attribute.
Yahoo answers is great for traffic though! Yahoo answer not give you dofollow link. Already said @SearchBliss
There are only nofollow links in Yahoo Answers. However, you need nofollow links too. Traffic can come in but you should not rely on that. Be careful since it is really easy to appear as a spammer in Yahoo Answers if you keep answering with your domain name as a resource. You have to get the best answers for many questions and add different domains as references. Basically, you have to actually answer the questions and be 100% honest. Help the people and only add your backlink when it is 100% relevant. You see the 100% theme? That is what will help you with Yahoo answers.
You do not need a lot of time. You just need to search for questions that you can answer really well and go for the top answer.
Upon checking, links (source/s) on Yahoo Answers are nofollow links. Even if you are a reputable/contributor for a long time, all the links shared are set to nofollow.