What is the spell that umbridge used on centaur in Harry potter 5?

Umbridge didn't use a spell on the centaurs in Harry Potter and the Order of the Phoenix. She used a Mandrake cry as a weapon against them.

While not strictly a spell, the cry of a mature Mandrake is extremely powerful, capable of rendering anyone who hears it unconscious. Umbridge, believing the centaurs were a threat, used the cry to incapacitate them.