This is the counter to the previous play with the lance, when one rides against the other one with steel and one lance is shorter than the other. When the one with a short lance carries it in Posta Dente di Zenghiaro, the one with the long lance similarly brings his lance low, because the short lance cannot beat the long. This is shown here.
When two horsemen ride against each other, and both carry their lances in Posta Dente di Zenghiaro, the one with the longer lance will win the exchange. Each is trying to perform the play of the 1st Master, but the longer lance will arrive at its target first. This enables it to reach underneath the shorter lance and beat it out of the way.