Indexes hints in a Subquery - oracle

Question

I have a SQL statement that has performance issues.
Adding the following index and a SQL hint to use the index improves the performance 10 fold but I do not understand why.
BUS_ID is part of the primary key(T1.REF is the other part fo the key) and clustered index on the T1 table.
The T1 table has about 100,000 rows. BUS_ID has only 6 different values. Similarly the T1.STATUS column can only have a limited number of
possibilities and the majority of these(99%) will be the same value.
If I run the query without the hint(/*+ INDEX ( T1 T1_IDX1) NO_UNNEST */) it takes 5 seconds and with the hint it takes .5 seconds.
I don't understand how the index helps the subquery as T1.STATUS isn't used in any of the 'where' or 'join' clauses in the subquery.
What am I missing?
SELECT
/*+ NO_UNNEST */
t1.bus_id,
t1.ref,
t2.cust,
t3.cust_name,
t2.po_number,
t1.status_old,
t1.status,
t1.an_status
FROM t1
LEFT JOIN t2
ON t1.bus_id = t2.bus_id
AND t1.ref = t2.ref
JOIN t3
ON t3.cust = t2.cust
AND t3.bus_id = t2.bus_id
WHERE (
status IN ('A', 'B', 'C') AND status_old IN ('X', 'Y'))
AND EXISTS
( SELECT /*+ INDEX ( T1 T1_IDX1) NO_UNNEST */
*
FROM t1
WHERE ( EXISTS ( SELECT /*+ NO_UNNEST */
*
FROM t6
WHERE seq IN ( '0', '2' )
AND t1.bus_id = t6.bus_id)
OR (EXISTS
(SELECT /*+ NO_UNNEST */
*
FROM t6
WHERE seq = '1'
AND (an_status = 'Y'
OR
an_status = 'X')
AND t1.bus_id = t6.bus_id))
AND t2.ref = t1.ref))
AND USER IN ('FRED')
AND ( t2.status != '45'
AND t2.status != '20')
AND NOT EXISTS ( SELECT
/*+ NO_UNNEST */
*
FROM t4
WHERE EXISTS
(
SELECT
/*+ NO_UNNEST */
*
FROM t5
WHERE pd IN ( '1',
'0' )
AND appl = 'RYP'
AND appl_id IN ( 'RL100')
AND t4.id = t5.id)
AND t2.ref = p.ref
AND t2.bus_id = p.bus_id);
Edited to include Explain Plan and index.
Without Index hint
------------------------------------------------------|-------------------------------------
Operation | Options |Cost| # |Bytes | CPU Cost | IO COST
------------------------------------------------------|-------------------------------------
select statement | | 20 | 1 | 211 | 15534188 | 19 |
view | | 20 | 1 | 211 | 15534188 | 19 |
count | | | | | | |
view | | 20 | 1 | 198 | 15534188 | 19 |
sort | ORDER BY | 20 | 1 | 114 | 15534188 | 19 |
nested loops | | 7 | 1 | 114 | 62487 | 7 |
nested loops | | 7 | 1 | 114 | 62487 | 7 |
nested loops | | 6 | 1 | 84 | 53256 | 6 |
inlist iterator | | | | | | |
TABLE access t1 | INDEX ROWID | 4 | 1 | 29 | 36502 | 4 |
index-t1_idx#3 | RANGE SCAN | 3 | 1 | | 28686 | 3 |
TABLE access - t2 | INDEX ROWID | 2 | 1 | 55 | 16754 | 2 |
index t2_idx#0 | UNIQUE SCAN | 1 | 1 | | 9042 | 1 |
filter | | | | | | |
TABLE access-t1 | INDEX ROWID | 2 | 1 | 15 | 7433 | 2 |
TABLE access-t6 | INDEX ROWID | 3 | 1 | 4 | 23169 | 3 |
index-t6_idx#0 | UNIQUE RANGE SCAN | 1 | 3 | | 7721 | 1 |
filter | | | | | | |
TABLE access-t6 | INDEX ROWID | 2 | 2 | 8 | 15363 | 2 |
index-t6_idx#0 | UNIQUE RANGE SCAN | 1 | 3 | | 7521 | 1 |
index-t4_idx#1 | RANGE SCAN | 3 | 1 | 28 | 21584 | 3 |
inlist iterator | | | | | | |
index-t5_idx#1 | RANGE SCAN | 4 | 1 | 24 | 42929 | 4 |
index-t3_idx#0 | INDEX UNIQUE SCAN | 0 | 1 | | 1900 | 0 |
TABLE access-t3 | INDEX ROWID | 1 | 1 | 30 | 9231 | 1 |
--------------------------------------------------------------------------------------------
With Index hint
------------------------------------------------------|-------------------------------------
Operation | Options |Cost| # |Bytes | CPU Cost | IO COST
------------------------------------------------------|-------------------------------------
select statement | | 21 | 1 | 211 | 15549142 | 19 |
view | | 21 | 1 | 211 | 15549142 | 19 |
count | | | | | | |
view | | 21 | 1 | 198 | 15549142 | 19 |
sort | ORDER BY | 21 | 1 | 114 | 15549142 | 19 |
nested loops | | 7 | 1 | 114 | 62487 | 7 |
nested loops | | 7 | 1 | 114 | 62487 | 7 |
nested loops | | 6 | 1 | 84 | 53256 | 6 |
inlist iterator | | | | | | |
TABLE access t1 | INDEX ROWID | 4 | 1 | 29 | 36502 | 4 |
index-t1_idx#3 | RANGE SCAN | 3 | 1 | | 28686 | 3 |
TABLE access - t2 | INDEX ROWID | 2 | 1 | 55 | 16754 | 2 |
index t2_idx#0 | UNIQUE SCAN | 1 | 1 | | 9042 | 1 |
filter | | | | | | |
TABLE access-t1 | INDEX ROWID | 3 | 1 | 15 | 22387 | 2 |
index-t1_idx#1 | FULL SCAN | 2 |97k| | 14643 | |
TABLE access-t6 | INDEX ROWID | 3 | 1 | 4 | 23169 | 3 |
index-t6_idx#0 | UNIQUE RANGE SCAN | 1 | 3 | | 7721 | 1 |
filter | | | | | | |
TABLE access-t6 | INDEX ROWID | 2 | 2 | 8 | 15363 | 2 |
index-t6_idx#0 | UNIQUE RANGE SCAN | 1 | 3 | | 7521 | 1 |
index-t4_idx#1 | RANGE SCAN | 3 | 1 | 28 | 21584 | 3 |
inlist iterator | | | | | | |
index-t5_idx#1 | RANGE SCAN | 4 | 1 | 24 | 42929 | 4 |
index-t3_idx#0 | INDEX UNIQUE SCAN | 0 | 1 | | 1900 | 0 |
TABLE access-t3 | INDEX ROWID | 1 | 1 | 30 | 9231 | 1 |
--------------------------------------------------------------------------------------------
Table Index
CREATE INDEX T1_IDX#1 ON T1 (BUS_ID, STATUS)