@broadsword80
The idea is quite simple really and related to one of the things that no old-time general worth their salt wanted to do to an opposing army that was beaten.
Drive them into a corner, on one side there is guaranteed death either by trying to flee or not doing a thing, on the other side, maybe, just maybe, a chance to live by cutting through the oncoming army. The desperation drives the cornered person to fight harder than they otherwise would have, plus with what
@gameriuxlt said, if you have your trained troops behind them, you not only have a meat-shield for your valuable soldiers but there's a chance that the enemy troops would lose momentum going through the conscripts, leaving your men fresh and ready to fight a tired enemy.
Of course with everything that had happened with the army being relentlessly driven to the killing ground on top of being forced to abandon their fellow soldiers and conscripting barely trained peasants along the way (leaving villages without manpower in a situation that is already dangerous with a possible food shortage), this tactic is barely holding together as tired and frightened peasants don't make for good meat shields as they want to find safety somewhere. Hence the cavalry on both wings waiting until the right time to launch their attack, give the conscripts enough time to find avenues to flee and cause chaos amongst their number, then attack, firmly entrenching the need to flee rather than fight in the conscripts.