It's called "Co-Contraction.
In a nutshell:
Build Size & Strength With Isometric Co-Contractions: 4% Increase in Arm Circumference and ~20% More Strength With Less Than 10 Minutes of "Flexing" Per Week
...the idea of using simultaneous voluntary contractions of antagonistic muscle pairs (aka co-contraction) to improve muscle strength in the absence of external apparatuses is not new... its efficacy is ... backed by previous research and its working principle is as simple as ingenious: Biceps and triceps, i.e. elbow flexor and extensor, produce resistive forces that act against each other (Tyler. 1986), so that one muscle actually "trains" the other.
This is how it's done: In a standing position with the feet shoulder width apart, upper arm vertical to the ground, the elbow joint at 90° (full extension = 180°), forearm in a neutral (middle of supinated and pronated) position you will perform a 4-s muscle isometric co-contraction followed by 4-s muscle relaxation (total volume: 10 times per set, 5 sets per session; rest: 2 min between sets). Make sure to perform each voluntary co-contraction as rapidly as possible and sustain maximal effort for full 4s!
Basically ... summarize as 10x4s maximal isometric muscle contractions with 4s break between each contraction for one set and 5 of these sets with an interim of 2 minutes between each of them three times per week.