This convention was made by Bjørn Olav Ekren, Norway, and has become very popular here in Norway.
The Ekrens 2D opening promises at least 4-4 in the major suits and 3-10 HCP. "Ekrens" is basically a preempt convention, but it should be used with care if you don't meet with these restrictions:
You should have your HCPs concentrated mostly in the major suits, you shold have maximum 5-5, and with 4-4 you should not have an invitational hand against 1NT opening.
(These restrictions are my personal opinion based on experience.)
Responses to 2D:
2H: To play.
2S: To play.
2NT: Forcing (Relay).
3C: To play.
3D: Invitational with 3-3 in the major suits.
Rebids after 2D-2NT:
3D: 5-5 in the major suits.
3H: Maximum with 5 Hearts and 4 Spades.
3S: Maximum with 5 Spades and 4 Hearts.
3NT: Maximum with 4-4.
Responses after 2D-2NT-3C:
3D: Repeated relay.
Rebids after 2D-2NT-3C-3D:
3H: 5 Hearts & 4 Spades.
3S: 5 Spades & 4 Hearts.
3NT: 4-4 in the major suit.
Note that after 2D-2NT(relay)-3D(5-5),
3H/3S is slam invitational: