Seen from above, the Lead Link is the person to whom a Circle assigns a set of accountabilities for him/her to organize. In this context, organizing means assigning roles: the Lead Link is the only one in the Circle who can assign roles. The Lead Link can do so at his discretion.
A Lead Link is the closest you can get to a traditional manager in Holacracy. He behaves similarly to a traditional manager in many things:
Establishing performance metrics
Maximising Circle performance
Monitoring Circle member performance
Defining Circle strategies
However, a Lead Link has only two unique tools to help him. First, he can reshuffle Role assignments if he believes moving Roles between team members will improve performance. Second, he can publish Strategies which serve as a guideline for each Circle member in prioritizing work. Other than that, the Lead Link can use the tactical and governance process just like any other team member may.
Filling a Lead Link Role is hardly ever a full-time job; usually, a Lead Link fulfills one or more 'regular' Roles in the Circle too, placing him far more in the midst of the team rather than above.
Contrary to the traditional manager, a Lead Link has far less authority. First, a Lead Link may only influence the prioritization of Circle Members' activities; he cannot step in and direct how Circle Members should work. Nor does the Lead Link have any explicit authority to 'sign off' or 'approve' any work done by Circle Members.
Note that, to safeguard the Circle from (accidentally) transforming the Lead Link to a traditional manager, no accountabilities may be added to the Lead Link Role. Period. For similar reasons, a Circle's Lead Link may not be elected as Facilitator or Rep Link. The Rep Link in itself is also meant to 'balance' the Lead Link; it is the Lead Link who represents a Circle in the Super-Circle. Think of the Rep Link as the 'upward channel' whereas the Lead Link is a 'downward channel'.
For the Lead Link, assigning Roles is an ongoing activity, and he can do so outside of Tactical and Governance meetings.
The Lead Link is assumed to fulfill any unassigned roles. Remember: The Lead Link is assigned with the accountabilities of the Circle, so if he opts not to assign a Role then who else is accountable?
A Circle's Lead Link is assigned by the Super-Circle's Lead Link. (seen from the Super-Circle, any Sub-Circle is a Role)
Relevant articles: 2.2, 2.4, 2.5.1, 2.6.3