When aiming for finger joint stability in playing piano one is surprisingly independent of accessory devices to strengthen up. In this film here (finger training device) finger plucking is practiced with a device which I DO NOT RECOMMEND in the WAY it is used!  The fingers are used in isolation and there is no support by the "knuckle" or MCP (metacarpo-phalangeal) joint. Instead, the flexor muscles of the finger (deep and superficial flexors or flexor digitorum profundus and superficialis) are brought through their entire range rather forcefully - which is not only NOT NECESSARY, but potentially detrimental. I used and altered the concept to show why we actually just need ourselves and a surface to coordinate this intricate interplay of joints in our hand:

Here's my short video on how to balance the interphalangeal joints either with a device (here I use a cup and some kitchen foil) or on a flat table top/ one's thigh. Note that this video does not contain a didactic chain on how to instruct a student how to get there, but only a short demonstration how little force and motion is involved. If you have a student, you need to take a number of preperatory steps and instruct carefully what you are aiming at. More to follow shortly!