Hold back is not a guaranteed make money deal, it is actually a way for the Manufacturer to ensure the dealer has some money in his Account with the manufacturer so he can pay his monthly bill.
The way it works is that when the dealer purchases the unit from the manufacturer he pays 3% more than the actual invoice cost, the manufacturer holds that money (without paying any interest) until the unit is sold and warranty registered. The manufacturer then pays the hold back to the dealer, usually semi-annually, sometimes annually, often during the "slow" months of the year, December or January. The money goes into the dealers parts account as a credit, then the manufacturer can collect what they are owed by the dealer and can remit any credit balance via check or ACH transfer.
In essence the dealer is getting his own money back with no interest after the manufacturer has used it for 6-12 plus months. Is it truly profit? Depends on the current cost of money and the dealers inventory turn and the timing of the payment from the manufacturer. It can be a big check and does help off season cash flow i can assure you.