From the Super Mario Wiki, the Mario encyclopedia
Lightning Spear is the Offensive Power Shot used by Shy Guy in Mario Power Tennis. When used, Shy Guy turns into a Spear Guy, and then does a chant in front of the ball. The screen turns dark, as a stream of lightning falls down to the ground, hitting the ball. This launches it into the air as a Lob Shot. However, a small star appears on the part of the ground it is going to land on, so the opponent can hit it back with ease. However, when any foes hit it back, they will get electrocuted for a small while, preventing them from moving around. Fly Guy's Special Shot in Mario Hoops 3-on-3, the Thunder Shot, might be based on the Lightning Spear.
It is extremely similar to Paratroopa's Offensive Shot, as both involve electricity, except their effects on the receiver are completely different.