The Rocker Depressor Tool is used to depress the closing rocker to obtain the loaded clearance when checking valve clearances.
The closing rocker has a simple cantalever spring under it to bias it closed. This is needed to allow the motor to idle smoothly. There is a time in the piston cycle (near Top Dead Center, TDC) when a slight gap, the clearance, is present between the closing rocker and the closing shim. The valve is completely closed but because of the small clearance can rattle open if a spring was not present. When the motor is running at higher rpm's the spring really has no function. In fact many race bikes simply remove the spring. But at idle, the spring is needed to guarantee the valve is completely closed around TDC.
To measure the closing clearance, you first measure the clearance between the opening rocker and the opening shim (opening clearance= unloaded clearance). You then press down on the closing rocker to take up the play in the closing rocker and overcome the spring force and measure the same clearance between the opening rocker and opener shim (= loaded clearance). The closing clearance = Loaded clearance- Unloaded clearance.
The Rocker Depressor Tool was designed to hook onto either side of the closing rocker arm that raps around the valve stem under the closing shim. Its hollow on the end that contacts the rocker and is hardened steel, but softer than the rocker so it will not damage it. It gets larger on the other end to fit nicely in the palm of your hand so you can push down easily to bias the closer spring. It makes it easy to hold the rocker arm down while at the same time measuring the Loaded Gap with a feeler gage.
The Rocker Depressor Tool can be used on all Ducatis that require the loaded/unloaded method of clearance measurement (2V and 4V bikes). It is not required on Testastretta motors (749, 998, 999, 848, 1098, 1198) since the closing clearance can be measured directly between the closing cam and closing rocker.