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After having a closer look, you don't even need a tangent mate, you just need to put some mate connectors in well placed spots to allow for a sliding mate paired with a revolution mate. The sliding mate could have limits so that when the actuator gets near the red arm, it starts to hit the slider limit in turn pushing the red arm the correct direction.

Will need access to copy the document for further guidance.

My bad, I was thinking that the sharing link allows writing. I've changed the document to public, so you should be able to play with it now. Let me know if it's ok.

I've tried various configuration of sliding edges but none has worked. But I'm far from an OnShape expert, having used it until now to model simple mechanisms composed of 3D printed parts.

Many thanks Michael for your quick reply.

Here is a link to the document : https://cad.onshape.com/documents/e611f9c9fe382489dc6c6b31/w/aa15001cdbadba9e845ac582/e/bca21fd677662a557f8ba01b?renderMode=0&uiState=6786e3a631048d13ede0ccc0

This is a triggering mechanism I'm adding to a double pendulum I've built for a student working on an experimentation project about the chaos theory.

The goal is to reproduce as exactly as possible identical initial configurations by placing the trigger at fixed positions (thanks to an additional external support) and observe that even then, due to unavoidable infinitesimal differences, the motion of the pendulum for several trials will be completely different. The experiments are recorded with a high speed camera (240 FPS) and a video tracking app (Tracker - https://physlets.org/tracker/) is used to record the trajectory, speed and accelerations of the red and green markers at the end of the arms.

Here is the full view of the trigger :

The yellow part is some sort of piston, used to hold the pendulum arm at the chosen initial positon by placing it in the end fork. Elastic bands attached to the hooks of the piston and the blue base plate pull so that it quickly retracts and releases the pendulum at experiment start. While armed, the piston is kept in place by the red hook. When the grey horn rotates, it pulls the hook up, which makes it release the piston.

This could seem a bit convoluted as a mechanism, and one could ask why the servo horn is not used directly to keep the piston in place. It was the option used in the first version of the mechanism, but it reveals that the constraints applied to the horn by the piston are too important and in a wrong direction, resulting in high chances to have something broken very quickly. In the current configuration, these constraints are all absorbed by the rotation axis of the red part.

I hope that these explanations make my goal easier to understand.

WRT to your remark about a partial tangent which would be active only when the parts are in contact, I'd add that I'm fine with a model where they are always in contact. But even then, I've not been able to model two perpendicular edges (one on the hook and one on the servo horn edges) sliding on each other.

Thanks again for you support.