Use of Case Statement in Index Creation

I was working on program and needed to access only Credit Memo applications to Invoices and Debit Memos. Query was something like

select 1 from ar.ar_receivable_applications_all , <other table1 > , <other table2 > .... .... where application_type = 'CM' and display = 'Y' and status = 'APP' and <other conditions>

In this query there was a join with many other tables and query was not performing well. Then i remembered recommendations of Lee Gunderson about usage of case statement in indexes. So i created following index and query worked very well

create index ar_receivable_applications_n80 on ar.ar_receivable_applications_all ( case when application_type = 'CM' and display = 'Y' and status = 'APP' then 'Y' else null end ) ;

After creating the above index, i rewrote the query as given below and performance improved immediately.

select 1 from ar.ar_receivable_applications_all where case when ra.application_type = 'CM' and ra.display = 'Y' and ra.status = 'APP' then 'Y' else null end = 'Y' and <other conditions>