Normally the bulbus glandis of the male holds the two dogs together as the female locks down over this portion of the penis shaft and the bulbus glandis swells creating the 'tie'. This is why it is important to never try to separate dogs who are tied -- the male can be injured.
Sometimes the male is so excited the bulbus glandis swells before the female is entered. When this happens he can't do a full tie as the swollen area is not within the gripping portion of the female's genital tract.
The important thing in breeding is that the sperm get into the reproductive tract of the female.
Holding the two dogs joined even if the bulbus glandis is outside the female can work to accomplish that.
