Heart-and-hand is an old Wraith ritual left over from the wars with the Ancients. It is a terrible and dangerous intimacy of blood and mind.
During the Lantean-Wraith wars, there had been enough different battles on different fronts that blades negotiated for their queens. The ritual has two blades lay their feeding hands on each other's hearts and set claws, skin to skin and life forces mingling.
The ritual doesn't prevent lying, as a strong-minded blade can make the weaker blade see what he wished, but it is difficult to do so. (SGA: "Lost Queen")