The Lock / Unlock Account View button allows you to switch between the activity dashboard and the account you are working on without having to filter for the account on the dashboard or the accounts page. A good time to use this is when you know you are going to be working on a single account for a period of time, locking the account will show only activity cards that are associated with that account on the activity dashboard as well as allowing you to not lose your place on whatever account page you were working on.
This button can be found in multiple places.
fig. 1. Top left below the account name on the account page
fig 2. From the activity dashboard, top right
fig 3. After clicking the lock account view on the activity dashboard, this window will appear, type in the account you are wanting to lock and view.