Only Apply To Unassigned Contacts
The first thing to note is the "Only apply to unassigned contacts" toggle. If this is OFF and the trigger fires for a contact who has already been assigned to a User, the trigger will overwrite the previous assignment.
Single User Assignment
If you select just one User from the "Select User" dropdown, the Contact associated with the trigger condition will be assigned that specific User every time the trigger fires.
Round-Robin Assignment
The trigger will round-robin assign contacts if you select more than one user from the Select User dropdown. For example, if you select two users (Tom and Jane); the first time the trigger fires it will add the contact to Tom, the second time it triggers it will assign the contact to Jane, the third time the trigger fires it will assign Tom, and so on and so on.
Weighted Round-Robin Assignment
Under the label "Split Traffic", a dropdown menu appears when more than one user is selected. By default, it is set to split traffic equally as described in the example above. When you change the dropdown to "Unevenly", you will see several more fields. According to the value you enter next to each person's name, you can weight the leads distribution unevenly.
The value you enter indicates how many leads should be assigned to that user before moving on to the next user.
In the scenario in the image above, the first time the trigger fires, the contact would be assigned to Chase. The second, third, and fourth times the trigger fires, the leads are given to David. The 5th - 9th times the trigger fires, the leads are given to Ian. The 10th time the trigger fires, it loops back around and the 10th contact would be assigned to Chase again.