Use the runes above when you know you can do the E+W combo for knockups. If not try warming up with going more tank with 9 Armor Glyphs and HP per level Quints.

When using support Rek'Sai
PROS:
-Very early engage ability and kill potential in lane.
-Very great for combos with Yasuo mid/top and VERY good with Kalista ult.

CONS:
-Potential to steal kills/farm if your ADC is not paying attention.
-You bring attention to yourself and could be the center of focus in team fights and ganks when you engage.

When going Rek'Sai bot try your best to have a Kalista with you due to her soul bound bonus and her ult. When she hits level 6 try to stay burrowed when you are ulted by her and focus the back line and when you explode in make sure your click instantly on one of the knocked up champs to knock them up again for even harder CC lock down. This by itself makes Rek'Sai one of the best supports for Kalista. Also try Vi if Rek'Sai is banned or Rek'Sai is picked away. Alot of the same ideas (Build on Vi soon)