Mutieren is the term for a low thrust following a movement around the blade of an opponent, having first met them with a preceding strike.
It is a response to a WEAK parry/bind.
The typical interpretation is seen in the application below.
|Vom tag, left leg forward||Vom tag, left leg forward, at measure|
|At once cut an Oberhauw to the upper left opening with a Passing Step|
|Parry but remain Weak(1) with the blade||Act Indes, wind the short edge on their sword over their blade to (their) right side, keeping the hands up high. Thrust down over his blade from this outside position to his lower opening so that your crossguard remains protecting your head and you stand in a posture similar to Ochs.|
(1) Whether the usage of "Weak" here refers to the Strong of the blade, or to an act of remaining physically strong in the bind is unclear, as the two are often used throughout the text. Experimentation suggest the former as this is what leads use to wind around with mechanical advantage - had it simply been a weak parry we would just thrust, as described in the Zornhau application.
Finally we notice that in the example application given (based on the von Danzig description) the movement is a follow on to our own opening cut. As with the Duplieren this technique can be used whether you are initiating the attack or not.
Not a specific technique in Meyer, rather the term mutating is used simply when moving from one mode of attack to another along different lines/openings, or changing from cuts to thrusts and vice versa.