Use in repairing wires
You don't seem to have identified the type of connector you are asking about.

There are rated and approved splice kits suitable for splicing NM cable inside the wall without a junction box.

For example, look at this one: http://www.homedepot.com/p/Tyco-Electronics-Romex-Splice-Kit-2-Wire-1-Clam-CPGI-1116377-2/202204326.

Note the strain relief on both sides of the splice, and the enclosed nature of the connection. But the real point is that it is legally approved for this use.