The Mishna says that contracts that are done in secular courts and signed by non-jewish witnesses, are bidning even between 2 jews. The gemara explains that by contracts of proof that do not effectuate an acquisition we have an umdana to trust the court since it would not have written a contract unless the sale was done properly (Tosafos 9b and 10b) assumes that even this is only binding through a special takanas chachamim. However, a contract that actually makes the acquisition go into effect and causes a change of ownership, such as a gift contract, should not have the ability to work. The gemara concludes either that it works through dina d'malchusa dina or that in fact it doesn't work. However, there should be a different method that would make a gift contract effective, namely, hoda'as ba'al din. Tosafos writes matter of factly that hoda'as ba'al din doesn't apply in this situation. Why? The Rashash is bothered by this and suggests that perhaps in the secular courts the giver doesn't give over the contract (which would be inherently admitting that he is passing on ownership), rather he testifies before a court and the court writes the documents, so without dina d'malchusa there isn't any halachically acceptable kinyan to make the property change hands.