For better understanding first look what server is doing.

DROP SCHEMA IF EXISTS indextest;
CREATE SCHEMA indextest CHARSET UTF8;
USE indextest;

CREATE TABLE t(i INTEGER, j INTEGER, PRIMARY KEY(i))ENGINE=INNODB;

-- Test without specials:

CREATE TABLE w(i INTEGER, j INTEGER, fkt INTEGER, PRIMARY KEY(i))ENGINE=INNODB;

ALTER TABLE w ADD FOREIGN KEY www (fkt) REFERENCES t(i);

SHOW CREATE TABLE w;

/* You will see that InnoDB created an 
index named www and
a constraint named w_ibfk_1.

Innodb only take the given name for the index
not for the constraint.
*/

-- What happens when index already exist?

CREATE TABLE x(i INTEGER, j INTEGER, fkt INTEGER, PRIMARY KEY(i))ENGINE=INNODB;

CREATE UNIQUE INDEX existing_x_index ON x(fkt);

ALTER TABLE x ADD FOREIGN KEY other_indexname_for_x (fkt) REFERENCES t(i);

SHOW CREATE TABLE x;

/*
You will see:
UNIQUE KEY `existing_x_index` (`fkt`),
CONSTRAINT `x_ibfk_1` FOREIGN KEY (`fkt`) REFERENCES `t` (`i`)

This means the name 'other_indexname_for_x' totally got ignored.
*/

-- What happens when existing index isn't unique?

CREATE TABLE y(i INTEGER, j INTEGER, fkt INTEGER, PRIMARY KEY(i))ENGINE=INNODB;

CREATE INDEX existing_y_index ON y(fkt);

ALTER TABLE y ADD FOREIGN KEY other_indexname_for_y (fkt) REFERENCES t(i);

SHOW CREATE TABLE y;


/* Result is same as on existing unique index */

-- existing combined indexes

-- Foreign key on first column in index list

CREATE TABLE z(i INTEGER, j INTEGER, fkt INTEGER, fkw INTEGER, PRIMARY KEY(i))ENGINE=INNODB;

CREATE UNIQUE INDEX existing_ind_z_fkt_fkw ON z(fkt, fkw);

ALTER TABLE z ADD FOREIGN KEY fk_first_column_in_index (fkt) REFERENCES t(i);

SHOW CREATE TABLE z;

/* You will see that no new index was created */

-- Foreign key in second column in index list

ALTER TABLE z ADD FOREIGN KEY fk_scnd_column_in_index (fkw) REFERENCES w(i);

SHOW CREATE TABLE z;

/* You will see that InnoDB created an index for column fkw */