An assembly for creating a sidetrack in a wellbore using a downhole tool with a whipstock, the assembly being run on a drillstring communicating fluid flow, the assembly comprising: a housing for a milling tool configured to support the downhole tool extending therefrom, the milling tool configured to connect to the drillstring and having a mill, the housing for the milling tool defining a bore therethrough communicating the fluid flow from the drillstring to the mill, the bore having an uphole shoulder and having a port communicating the bore outside the housing; a piston movable from a closed state in an uphole position toward an opened state in a downhole position in the bore, the piston having uphole and downhole ends and defining a passageway therethrough, the piston having at least one channel (127, 144) defined in an outer surface of the piston, the passageway defining an uphole-facing surface area exposed to the fluid flow, the piston in the uphole position closing the port from the bore, the piston moved from the uphole position toward the downhole position exposing the port to the fluid flow in the bore, the port disposed in fluid communication with the downhole tool and being configured to communicate pressure from the fluid flow in the bore to the downhole tool; and a releasable connection (140a-b) disposed in the at least one channel (127, 144) and having engaged and unengaged states with respect to an edge (DFE, UFE) of the at least one channel (127, 144) of the piston in the closed state, the releasable connection (140a-b) in the engaged state being disposed in contact with the edge, the releasable connection (140a-b) in the unengaged state being disposed at a clearance distance from the edge, in response to a predetermined downhole force from the fluid flow in a downhole direction against the exposed surface area of the piston, the releasable connection (140a-b) being in the engaged state disposed in contact with the edge of the at least one channel (127, 144) of the piston and being configured to release the piston to move from the closed state in the uphole position toward the opened state in the downhole position, and in response to an uphole force from the fluid flow in an uphole direction against the piston in the closed state, the uphole end of the piston being configured to abut the uphole shoulder and the releasable connection (140a, 140b) being in the unengaged state disposed at the clearance distance from the edge of the at least one channel (127, 144) of the piston, whereby the releasable connection (140a, 140b) is unexposed to the uphole force. a housing for a milling tool configured to support the downhole tool extending therefrom, the milling tool configured to connect to the drillstring and having a mill, the housing for the milling tool defining a bore therethrough communicating the fluid flow from the drillstring to the mill, the bore having an uphole shoulder and having a port communicating the bore outside the housing; a piston movable from a closed state in an uphole position toward an opened state in a downhole position in the bore, the piston having uphole and downhole ends and defining a passageway therethrough, the piston having at least one channel (127, 144) defined in an outer surface of the piston, the passageway defining an uphole-facing surface area exposed to the fluid flow, the piston in the uphole position closing the port from the bore, the piston moved from the uphole position toward the downhole position exposing the port to the fluid flow in the bore, the port disposed in fluid communication with the downhole tool and being configured to communicate pressure from the fluid flow in the bore to the downhole tool; and a releasable connection (140a-b) disposed in the at least one channel (127, 144) and having engaged and unengaged states with respect to an edge (DFE, UFE) of the at least one channel (127, 144) of the piston in the closed state, the releasable connection (140a-b) in the engaged state being disposed in contact with the edge, the releasable connection (140a-b) in the unengaged state being disposed at a clearance distance from the edge, in response to a predetermined downhole force from the fluid flow in a downhole direction against the exposed surface area of the piston, the releasable connection (140a-b) being in the engaged state disposed in contact with the edge of the at least one channel (127, 144) of the piston and being configured to release the piston to move from the closed state in the uphole position toward the opened state in the downhole position, and in response to an uphole force from the fluid flow in an uphole direction against the piston in the closed state, the uphole end of the piston being configured to abut the uphole shoulder and the releasable connection (140a, 140b) being in the unengaged state disposed at the clearance distance from the edge of the at least one channel (127, 144) of the piston, whereby the releasable connection (140a, 140b) is unexposed to the uphole force.