Meet pylon flaps:
They are hydraulically powered, and are activated when the yoke is pushed near full forward position. They aid in recovery from nose-high attitudes. Presumably, the increased weight of the IAE V2500 engines in back makes nose-high recovery a bit more challenging than the MD-80 series with the comparatively lighter JT8D engines.
More views - from the side, from behind, and from below.
I'd love to see some photos of MD-90 pylon flaps deployed. Anyone out there have access to an MD-90 maintenance hangar? There may be beer in it for you.