"Peruvian Necktie" refers to an arm triangle choke from the sprawl position, with one leg over uke's head and the other over their back:1
The IJF rules ban arm-triangles which "block the opponent's body with the legs", and alhough the example picture2 for this rule shows a do-jime/closed guard position (as opposed to the Peruvian necktie 'side-on' position), the wording is a big vague.
Is the Peruvian necktie competition legal?
This term is also sometimes used to refer to the "Gerbi" choke, a similar choke applied with tori's gi-skirt.
Kata-sankaku grip situation in ne-waza: it is prohibited to block the opponent's body with the legs and mate must be announced (picture 2).