How we can enable `index_condition_pushdown`?
index_condition_pushdown is used for Queries with the range, ref, eq_ref, and ref_or_null access methods, i.e when there is a need to access full table rows.

Where as Index Condition Pushdown is enabled by default; it can be controlled with the optimizer_switch system variable by setting the index_condition_pushdown flag.

To Switch:

 SET [GLOBAL|SESSION] optimizer_switch='command[,command]...';


for more you could read here:
http://dev.mysql.com/doc/refman/5.6/en/index-condition-pushdown-optimization.html

and http://dev.mysql.com/doc/refman/5.6/en/switchable-optimizations.html