The public class variable
$enable_sticky_sessions pins the execution of commands to the last connection used. The default value for this is true.
The normal behavior of the load balancer is to re-use the first found connection, as the overhead of establishing a database connection is generally larger than that of running multiple simple queries against a single connection.
The primary use-case for disabling this is if you are running a substantial number of heavy queries and want to truly load balance at the per-query level rather than the per-request level.
Since connecting to the database can take longer than running a few queries, it is less efficient to startup 5 connections to run 5 separate queries, when all 5 queries could be run on a single connection in less time. However if you need to run 1000 queries in a row (ie: batch job), it might be better to startup 5 connections and load balance all 1000 queries across those connections.
$db->enable_sticky_sessions = false;