Term given to partnership agreement about entering the bidding after an opponent’s Multicoloured Two Diamond opener.
In second seat, you expect to get two chances to bid, which increases your options. You can directly bid 2♥, 2♠, 3♣ or 3♦ as natural, 2NT to show a strong no- trump.
Double is usually two-way, showing either a balanced hand too weak to bid 2NT or any strong hand. In sixth seat (i.e. when you are in second seat but pass first time), double of 2♥/2♠ is for takeout.
In fourth seat, a simple method is to treat the auction 2♦(Multi)-pass-2♥ the same as 2♥(weak)-pass-pass and 2♦(Multi)-pass-2♠ the same as 2♠(weak)-pass-pass.