On the Experiment summary page you find the option to change the traffic distribution of an experiment.
The distribution allows you to control what percentage of visitors you like to test. In the example above you can see that 50% of the visitors will not take part in the experiment and will see the original version.
Their behavior will not be monitored. The test group of 50% can then be divided as you like between the original and the variations. You can customize the variation traffic distribution as per your preference.