Let me tell you how BDs infiltrate into India in areas which are fenced. The system is called tangra system. Tangra is the code for the would-be BD migrant.
Anyway.
The border fencing system uses two parallel fences of barbwire with a roof of a barbwire mesh. Now a would-be migrant, i.e. the tangra, pays an amount to an agent in BD and I'm told that the amount varies. This agent then pays to his counterpart in India, through hawla. Sometimes they pay off the security forces on either side of the border, but most of the times, to make more profit, one or both of the agents may just neglect to do that. Once the cash transaction is complete, the BD agent brings a couple of bamboo ladders in the middle of the night. One is placed against the fence on BD's side to climb up, while the other is placed across the barbwire mesh at the top, forming a bridge between the two parallel fences. The would-be migrant, after reaching the top, pulls up the 1st ladder and places it against fence on the Indian side to climb down. Once on the other side, if the tangra is lucky, s/he will be given a fake ration card and s/he will forever vanish in India, becoming *cough* *cough* a Bengali Muslim (however, introducing him/herself as Hindu). If its a she, and is unlucky, she will find herself in a brothel somewhere in India.
If the security forces are paid off, they will simply not patrol that area during the fixed time. If they are not paid off and detect such trespassing they will just shoot to kill (recall that shooting incidence a couple or so months ago where a young BD girl was shot dead).