Aha, now I see. Lender makes a btc transaction to the borrower. So, no longer the lender's keys. This checks out.

If the btc are not back by the given time, the collateral is released (to the lender, I suppose). So the risk would be an insolvent borrower AND the collateral not covering the "new" value of btc at loan expiry. This also checks out.

What does not check out is the following. Imagine being the borrower. Imagine being asked for, say, 200% collateral. Maybe in USDC/USDT? OK. So at expiry, if the btc you borrowed is worth more than the collateral, why give the loan back? Let the lender just have the collateral. On the contrary, if the btc you borrowed is worth less than half the collateral, give it back of course, and keep your relatively precious collateral. In other words: a shorter's paradise. (EDIT - No: it's more like an arbitrage, since it's impossible to lose if this is all there is to it.) Am I missing something? It's obvious I am, but I don't know exactly what - or I wouldn't be missing it  

Another thing that escapes me: if you can post, say, 100k usd collateral, why would you borrow a bitcoin rather than buying one - or two? For simplicity, I'm assuming 1btc = 50k$.

Biodom, have you been able to figure this one out?