What is a Straddle?
The player under the gun can choose to make a preflop raise of two times the big blind. The UTG+1 player has to act first preflop if the UTG player posts a straddle. It is like posting a third blind, letting you act last preflop.
When can you Straddle?
The straddle must be made at the same time the blinds are posted. So you have to decide whether or not you want to straddle before any cards are being dealt.
On all poker rooms of the GGNetwork you can enable straddling with a button switch in the poker client. If straddling is enabled, you will post the straddle each time you are in the right position to do so.