Can you explain this for the people who don't know what this is? I mean, I obviously do. But I feel like maybe some other people don't and I want them to know.
Sure.
So, first rule is that you NEVER want to put any money down on a lease (with the exception I'm going to note below). When you see lease specials advertised with, say, $2499 due at signing, you will always want to change that amount due at signing and roll it into the lease. Yes, the payments will be higher, but putting money down on a lease is always a bad idea. If anything happens to the car during the lease (stolen, totaled, etc), you do not get any of that money back.
For simplicity sake, if your lease payment is $100/month and it's a 36 month lease, you will end up paying $3,600 total over the time of the lease. You can give them a check for $3,600 the day you get your car and not have any payments remaining. If your car gets totaled the next day, you have no car and you do not get that $3600 back. If you had only paid the $100 for the first month and it gets totaled, you get to walk away from the lease deal without owing any more money.
The exception to that is putting down a security deposit. This is to cover any costs at the end of the lease (excess miles, excess wear and tear, fines, etc.). If there are no costs, you get that money back. If the car is totaled or stolen, you also get that money back.
The MSD program allows you to put down multiple security deposits. They are equal to one month's payment and BMW allows you to put down 7 of them. What they do, in return, is lower the money factor/interest rate on the lease. So, for example, a lease payment of $550/month will get dropped down to maybe $515/month. You are now saving $35/month x 36 months = $1,260 over the 3 year span of the lease.
The cost to do that is giving them $550 x 7 = $3,850. When you sign the lease, you give them $3,850 that will be returned to you at lease end (minus whatever costs you might incur). But you've saved $1,260 by letting them hold that money for you.
If you invested $3850 in an account, you'd have to earn almost 10% annually in order to make $1,260.
In other words, it's a no brainer unless you don't have that money around to spare.
ETA -- MSD programs aren't really advertised. You have to ask about it. And, in general, ask about it at the end after terms are negotiated.