Backward index scan is not reflected in EXPLAIN output

EXPLAIN SELECT * FROM t1 ORDER BY a ASC, b DESC;

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra

1	SIMPLE	t1	NULL	index	NULL	a_desc_b_asc	10	NULL	2	100.00	Backward index scan; Using index

Warnings:

Note	1003	/* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1` order by `test`.`t1`.`a`,`test`.`t1`.`b` desc

FLUSH STATUS;

SELECT * FROM t1 ORDER BY a ASC, b DESC;

a	b

1	2

2	1

SHOW STATUS LIKE 'Handler_read_prev';

Variable_name	Value

Handler_read_prev	2

EXPLAIN EXTENDED SELECT * FROM t1 ORDER BY a ASC, b DESC;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra

1	SIMPLE	t1	index	NULL	a_desc_b_asc	10	NULL	2	100.00	Using index

Warnings:

Note	1003	select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1` order by `test`.`t1`.`a`,`test`.`t1`.`b` desc

FLUSH STATUS;

SELECT * FROM t1 ORDER BY a ASC, b DESC;

a	b

1	2

2	1

SHOW STATUS LIKE 'Handler_read_prev';

Variable_name	Value

Handler_read_prev	2