As Dion pointed out, the seller can not "guarantee" a bail out of your funds if the loan defaults, but he can structure the contract in a way that he has first right to purchase the note back from you in a case of default. If you trust your seller, this would relieve the hassle of having to go through a foreclosure process.
As far as being "active" or "passive" in this investment, the loan servicing company is the only one doing any work. They are collecting the payments (and hopefully the insurance and tax escrows). They will notify you of the monthly payment or the lack of monthly payment. The home owner is responsible for all aspects of the actual property. Hopefully you are being "active" in the investment by seeing the valuation of the property and payment history before you make the investment.
One thing to be aware of is the originator of the note. If an investor is originating more than 5 notes per year, then a RMLO needs to oversee the process. Otherwise, there could be Dodd-Frank implications which could effect the actual note.
I believe partial notes are a good way for investors to place smaller chunks of money with a decent return. It also keeps their investment to asset value in a more comfortable range. Many times you can find a placement where your investment in the partial will be at 50-75% of the overall value of the real estate being used in the deed of trust/mortgage.
