The gemarra (Brachos 7a) writes that Bilam knew the exact minute fraction of a second that HaShem gets angry each day. It was at this point that Bilam would utter his curse, which would normally thus ‘work’ to pour damage upon the subject of his curse. However, when it came to cursing Klal Yisrael, HaShem performed a special kindness for us and closed up this window of anger.
Tosafos on the above gemarra asks the question; what on earth could Bilam have said in a fraction of a second? Tosafos gives two answers.
Either Bilam could have simply said kal’em (destroy them), or alternatively, once Bilam began his curse within that fraction of a second then anything he said subsequently would also be effective.
