The definition of collection is a combination of three factors:
 Engagement of the hindquarters, which means that the pelvis tips down (tail down) and in (rotating the bottom of the pelvis in and under the horse), bringing both legs under the mass of the horse.
 Bascule (rounding) of the back. Engagement of the hindquarters must happen for the back to round up.
 Lifting of the base of the neck. This allows the neck to telescope and the head to freely offer it's relaxed position slightly in front of the vehicle.
In tolt, the pelvis is doing the opposite of the engaged position; it is tipping up (tail up) and out (rotating the bottom of the pelvis up and out away from the mass of the horse).
This action, starts the back to hollow. The back cannot "round" when the hindquarters have not been engaged.
In collection, the lowering of the pelvis (engagement of the hindquarters) is a *sustained* lowering.
Sustained lowering never happens in tolt.
Going to the front of the horse: lifting the head and neck lowers the base of the neck, which makes tolt an earth-bound gait. The vector of the movement is down; earth-bound, which is the opposite of collection.