Stocks held in escrow account to meet 20% down conventional?

[Michael Zagorsky]

So I know someone who mentioned that his father in-law, who works for a investment bank, managed to get him an interesting mortgage for his primary residence.  He says that it is conforming mortgage.

Basically, instead of putting 20% down into equity on the property, he put 20% in the form of equities into a escrow account.  There were rules where the bank could call for additional capital to be paid in the stock value dropped below 20% and he would get his additional earnings out if the portfolio appreciated.

Anyone know anything about what these arrangements are called so I can read up more on them and if they exist for investment properties?