Top-down planning enables setting high level goals to a metric, say Sales $ and spreading the updated value to lower levels in the hierarchy while keeping penetration (i.e. % of TTL) of each level the same.
Say you want to grow the total Sales $ for April by 10%. You likely want to keep the contribution of each level in the hierarchy consistent. Top-down planning and spreading allows you to set the new total for Sales $ April and spread the expected sales to each level in the hierarchy proportionally. See below for how it works in action: