I wasn't really sure where to post this question but as I would be performing it in strolling sitiuations I've put it here so mods feel free to move it where you think it needs putting if not here.

For those that perform knife through jacket/coat how do you practice it ?
I know that sounds strange so I'll elaborate it....when performing this effect you need at least one spectator to hold the jacket in place so how do you go about that as when I practice it's alone until it's almost ready for the real world where I then do a few unpolished performances which I call active practice .

You could get a couple of spring clamps, the ones that look like pliers with springs, and clamp each shoulder to a dowel rod, shower curtain rod, a taught rope, clothesline, etc. Clamping each shoulder should somewhat accurately simulate the volunteer holding the jacket and provide a more realistic feel for how the jacket will behave.

Hope that helps with some ideas. Let us know what you choose for a solution.

Use 2 large bulldog clips, clip it between 2 chairs.

You could just use an open backed chair.