The JM Press is a hybrid, a monster. Adding chains and doing it with a far bar, like Dave does in this video, is akin to creating a weight room Frankenstein. This mutant exercise gets it done though.  This movement is a version of the standard JM Press.  If you aren't familiar with the JM Press, I suggest you learn it before attempting a modified version such as this.  The floor may limit the range of motion a little, but that's the point.  The chains will make the lockout much more difficult, which is also the point.  This is a great exercise to improve lockout power and general awesomeness.