Odwrotka is an artificial game-forcing raise of 3+ card support.
I propose two sets of responses to Odwrotka.
The simple 445566 structure is popular in Poland. I revert the strong and weak
steps to prevent the weak hand from declaring notrump via bidding 2NT.
1♣ -1M 2♦ - FG , 3+ SUPP
2♥ ! S/T , 10+, 4=M2♠ ! MIN , 7–10, 4=M2NT! S/T , 10+, 5=M3♣ ! MIN , 7–10, 5=M3♦ ! S/T , 10+, 6+M3M MIN , 7–9, 6+M
I recommend the relay structure in Polish Club 2020: Expert . The 2♥ response
to Odwrotka is the strongest step combining all slam tries. The relay structure
generally lets the strong opener declare notrump.
1♣ -1M 2♦ - FG , 3+ SUPP
2♥ ! S/T , 10+2♠ ! NEG , 7–10, 4=M2NT! MIN , 7–10, 5=M, not BAL 3♣ Mild S/T , 9–11, 5+♣ , 4=M 3♦ Mild S/T , 9–11, 5+♦ , 4=M 3M! MIN , 7–10, BAL 5=M3OM! SPL , 7–9, 0–1#, 6+M3NT MIN , 7–9, 6+M4♣ ! SPL , 7–9, 0–1♣ , 6+M4♦ ! SPL , 7–9, 0–1♦ , 6+M
…-2♥ S/T , 10+
2♠ ! (R) , 3–5M2NT NAT , 3=M3♣ ♦ S/T , 5+#3M NAT S/T , 4+#3NT! BAL COG , 18–20, 4M333
…-2♥ 2♠ - (R)
2NT 4=M 3♣ 4+♣ , 5=M 3♦ 4+♦ , 5=M 3OM 4+#, 5=M 3M! BAL 5=M3NT! 6+M 4♣ ! SPL , 0–1♣ , 6+M4♦ ! SPL , 0–1♦ , 6+M
…-2♥ 2♠ -2NT 4=M
3♣ ! Ask for a side suit 3♣ -3X 4+# 3♣ -3M! 4+♣ 3♣ -3NT 4M(333)
I designed rebids other than the 2♠ relay for more descriptive auctions.
…-2♥ 2NT 3=M
3♣ 4+♣ , 5+M 3♦ 4+♦ , 5+M 3OM 4+#, 5+M 3M 5+M w/o side suit 3NT 4=M