Yield means at what point something will give way, or break under pressure. in most processes this is done by a pivot or fulcrum arm which is a mechanical unit for the application of yield pressure. The top of an oil pump is a fulcrum unit that when pressure is applied at one end, what is connected to the other end yields to the mechanical differential ratio and this then draws the oil up from under the ground. A torque multiplier also does the same but in a more compact form. I am trying to get something more general, but it would be great to have the complete sentence. In Jacks answer it states the "Mechanical yield mechanism" But I think you want to know what the mechanism is and how it works. therefore the mechanism would be the workings of the unit and how it achieves its application of yield (out put) force when a lesser force is applied as input. Most use the fulcrum type as it is simple, others use spool gears, and gear drives.. The sentence would be good, and more information as to what you wish to understand, or do you want a replacement phrase?