When driving the tuning pins into a grand piano, the pin block has to be supported from the underside. The reason for this is the way in which the pin block is manufactured. Generally speaking, the pin blocks in pianos are constructed of multiple layers of wood laminated together. In modern construction, most pin blocks are manufactured of up to 18-20 layers laminated together with phenolic resins. In early construction, pin blocks were 3 layers of much thicker materials over a maple plank.
In either case, while stringing procedures are completed, or when the tuning pins must be driven deeper into the pin block for increased torque ratings, the block must be supported from the underside. Without support, the bottom layers of the pin block laminations will be forced apart by the tuning pin being driven into the block from the top. Once this de-lamination occurs, the repair is to replace the pin block, which is a time consuming and costly mistake.
Below is a link for some photos of the traditional way to jack and support a pin block. Of course there are other ways to accomplish this task without the specialty machinist’s jacks; by using a series of wedge planks would be one example.
When jack supporting the pin block this puts excessive pressure on the key bed; sometimes with vintage instruments one does not know how strong the key bed is. In instruments that are older, it is also a good idea at times to support the key bed from underneath the piano to the floor. This can be done with short lumber and a small hydraulic jack.
Below is a link to a series of photos showing the tools and supplies required for supporting the grand piano pin block.