A company I used to work for used a factoring company and although it may have changed since this is what happened:-
Once a month we would send a report detailing
- Invoices raised (we still sent this out, did our own credit control etc)
- Payments recieved (these would be going into a bank account in the companies name but held in trust by the factoring comapny)
- Debt analysis (30, 60, 90+ days)
- Purchases
After 48 hours we would recieve funds based on 75% of the invoices raised (minus any that would/could be contra'd against purchases). They would clawback/withold funds based on any 90+ invoices (the company I worked for did no credit control until I started

)
They would also release a further 15% of any payments recieved. Keeping the remaining 10% as fees.
TBH it helped this companies cashflow enourmously, the company was unique in the area, just been severly mismanaged, therefore they could raise their prices to cover the fees. But I agree that once in there it is very difficult to come off, unless the company can survive without any incoming cash for a month or two.
When we started up our business, my partner was keen to go down this route to help with the initial cashflow every new business has, however after questioning me more and doing some delving we decided not to.