Basically you pay a fee and the bond company goes guarantor for that amount, at the end of the bond period you pay that amount to the builder. If you dont the bond company pay the builder and they come after you for the money.
For example;
House cost €500K, you pay the booking deposit of say €5k but the 10% (€50K) is now due on signing contracts and you dont have available cash (tied up in a house for sale etc) so you take out a bond for €50k. For this amount it costs roughly €1000 in fees to cover a period of 4 months, the longer the duration the higher the cost. The auctioneer will then return your €5k as he now has a bond for the 10%. On close of the sale you give hand over 100% of the house cost, the bond just lapses. if you pull out and default on the deal the bond companies solicitors have their wicked way with you!