The “sewing machine jail shanking” is something way more popular in Aikido circles than in professional ends such as Law Enforcement or Special Operations. I would suggest that such popularity stems from an overall ignorance in both knife lethality and in prison/jail culture. This ignorance, I would also suggest, stems from Aikido’s main demographic’s cultural makeup, one wherein it has become over dependent upon the creation of a violence specialist class, such as peace officers or servicemen, to keep them safe. In the absence of relevant knowledge and experience, a fear of the Other (ie. the incarcerated) has combined with Hollywood myth-making to make modern Aikidoka prioritize the imagine jail shanking as the par excellence "realistic" knife attack and retreating as the only viable tactical response.
In truth, the jail shanking is more of a social violence than it is of an asocial violence. In that regard, its tactics are not solely determined by the martial neutralization of the target. Such acts of social violence are also determined largely by rules of engagement, social capital markets, and cultural semantics. These things, in conjunction with technological limitations, have gone on to shape related weapon selection, and weapon selection has gone on to concentrically select the “sewing machine” tactic, such that the martial neutralization of the target is (again stated) not the primary goal. The end result, and quite contrary to the growing mythology within Aikido circles, is that the “sewing machine jail shanking” is done primarily for cultural or social reasons and not for reasons of martial viability, and it is done with weapons that while fully capable of fulfilling said cultural or social ends are seldom, if ever, capable of doing more (ie. martially neutralizing a target).
Meaning, the “sewing machine jail shanking” is the antithesis of an asocial lethal knife attack, and it makes use of weapons that are themselves the antithesis of a truly lethal weapon. It is the latter that allows for a repeated in-and-out quick succession of stabs, and it is the former that selected for such inferior weapons capable of such stabs. In this regard, traditional and/or conventional Aikido understandings of knife attacks, such as the use of a blade approximately 6 inches in length targeting organs or major veins and arteries with a degree of penetration marked by spinal displacement, are the more lethal and more “realistic” - the more dangerous - knife attack between the two tactics, and this is something every violence specialist having a professional end to his/her art knows.
* Pictures above are two shanks - both representative of weapon type common to custodial facilities.