In a PoS system each node that holds a balance in the main network token has a chance (proportional to its balance) to produce a block. In the two-tier architecture it is logical to move payment processing onto the full nodes alone. At the same time, all nodes with non-zero balances still have to be eligible for staking rewards.
The theoretical issue of reduced security caused by fewer nodes staking can be addressed through explicit balance leasing from lightweight nodes to full nodes. By leasing their balance to a trusted full node a lightweight node actually increases its chance of collecting transaction fees, since it does not have to stay online, and the full node has an increased chance of producing a block due to its increased balance.
I'm posting this again since I think this was in the whitepaper
I feel like another ICO had a similar idea but I forget the name if anybody can help me.