A hydrokinetic coupling of the toroidal fluid flow type having an impeller member and a turbine member. The impeller member has a radially inwardly facing trough positioned radially outward of the turbine member which has a radially outwardly facing trough. The troughs together form a toroid of substantially elliptical cross section. A plurality of mutually facing stepped blades circumferentially disposed in the impeller and turbine member troughs define a stepped annular gap between the members. The gap is generally Z-shaped and includes two gap portions parallel to the rotational axis of the members radially offset by a third gap portion transverse to the rotational axis. An annular core guide ring is disposed in the transverse gap portion and secured to the impeller blades; this ring cooperates with a cylindrical sleeve, which is selectively movable into one of the offset gaps, to control fluid circulation between the impeller and the turbine. An annular split seal, carried by a portion of the impeller member, cooperates with the cylindrical sleeve to prevent fluid leakage from the impeller when the cylindrical sleeve is fully inserted into the offset gap and seated against the core guide ring.