A sword was planted in the Beth Hamidrash and it was proclaimed, ‘He who would enter, let him enter, but he who would depart, let him not depart!’
Presumably so that none of the voting members would leave and change the outcome of the vote.
The battle ends, the vote is taken:
And on that day Hillel sat submissive before Shammai, like one of the disciples, and it was as grievous to Israel as the day when the [golden] calf was made.
Ok, surely hyperbole. But a pretty dramatic rendition! Especially since the writers are followers of Hillel.