this can put more light here

CREATE LOGIN login1 WITH PASSWORD = 'J345#$)thb';

CREATE LOGIN login2 WITH PASSWORD = 'Uor80$23b';

GO

CREATE USER user1 FOR LOGIN login1;

CREATE USER user2 FOR LOGIN login2;

GO

begin tran t

GRANT IMPERSONATE ON USER:: user2 TO user1;

SELECT SUSER_NAME(), USER_NAME();

EXECUTE AS user = 'user2';

REVERT;

GRANT IMPERSONATE ON USER:: user1 TO user2;

SELECT SUSER_NAME(), USER_NAME();

EXECUTE AS user = 'user1';

revert;

rollback tran t