the tool 100 wherein the ball seat 200 has been shifted to an enlarged diameter position as shown the annular shifting piston 150 has moved from a first lower to a second higher position relative to the ball seat due to fluid pressure acting on the piston surface 152 of the piston 150 consequently the space 153 has been reduced in volume in operation an upwardly facing shoulder 154 of the annular piston 150 that is in contact with a lower surface 212 of the castellations 202 has forced the ball seat 200 with its castellations 202 upwards along the conical surface 210 thereby enlarging the inner diameter of the sealing ring 205 to a size exceeding the outer diameter of the ball 201 in this manner the ball is released and fluid communication is reestablished between the portions of the bore above and below the ball seat 200 in one embodiment the invention is practiced in the following manner a tool 100 including the ball 201 and ball seat 200 is run into a wellbore in a string of tubulars to a predetermined depth the ball seat is in its smaller diameter position as shown in fig1 however the bore through the tool is open because there is no ball in the seat during run in at some later.