If a half-wave dipole is converted to a Yagi by adding a slightly longer element, in what direction(s) does the radiation strength increase?

When a parasitic element is added to a Yagi antenna that is slightly longer (usually about 5% longer) than the main driven dipole, it acts as a 'reflector.' Just like a mirror reflects light, the longer element reflects the radio frequency energy back toward the driven element. This causes the main radiation lobe to push forward, away from the reflector and past the dipole. Consequently, the overall radiation strength increases in the direction starting from the new longer element and moving toward the main dipole.