How to host the Primary Domain from a subfolder using .htaccess

How do I make a sub directory (or sub folder) act as the public_html for your main domain?

# .htaccess main domain to subdirectory redirect
# Copy and paste the following code into the .htaccess file
# in the public_html folder of your hosting account
# make the changes to the file according to the instructions.
# Do not change this line.
RewriteEngine on
# Change yourdomain.com to be your main domain.
RewriteCond %{HTTP_HOST} ^(www.)?yourmaindomain.com$
# Change 'subdirectory' to be the directory you will use for your main domain.
RewriteCond %{REQUEST_URI} !^/subdirect