Can anyone explain this defence

[thepossum]

I appreciate maybe I knew the situation better than the bot

 

[HV=https://www.bridgebase.com/tools/handviewer.html?lin=st||pn|thepossum,~~M14222yh,~~M1779mjw,~~M18930p2|md|3SJT6HK8DAK32C8642,SQ43HQJ62DT84CA73,SAK85HAT753DQ75CQ,S972H94DJ96CKJT95|sv|o|rh||ah|Board%201|mb|1H|an|Major%20suit%20opening%20--%205+%20!H;%2011-21%20HCP;%2012-22%20total%20points%20|mb|P|mb|1N|an|Forcing%20one%20notrump%20--%203-%20!H;%203-%20!S;%206+%20HCP;%2012-%20total%20points|mb|P|mb|2D|an|New%20suit%20--%203+%20!D;%205+%20!H;%2011+%20HCP;%2012-18%20total%20points%20|mb|P|mb|2S|an|Impossible%20spade%20--%204+%20!D;%202-%20!H;%203-%20!S;%2011-12%20total%20points;%20forcing%20|mb|P|mb|2N|an|Invitational%20to%203NT%20game%20--%203+%20!D;%205+%20!H;%2015-17%20HCP;%2018-%20total%20points;%20partial%20stop%20in%20!C;%20partial%20stop%20in%20!S;%20forcing%20to%203D%20|mb|P|mb|3H|an|4+%20!D;%202%20!H;%203-%20!S;%2011-12%20total%20points|mb|P|mb|3S|an|3+%20!D;%205+%20!H;%203+%20!S;%2015-17%20HCP;%2018-%20total%20points;%20partial%20stop%20in%20!C;%20partial%20stop%20in%20!S%20|mb|P|mb|3N|an|4+%20!D;%202%20!H;%203-%20!S;%2010+%20HCP;%2011-12%20total%20points;%20partial%20stop%20in%20!C|mb|P|mb|P|mb|P|pc|C3|pc|CQ|pc|CK|pc|C2|pc|CJ|pc|C4|pc|C7|pc|H3|pc|C9|pc|C6|pc|CA|pc|H5|pc|S4|pc|SK|pc|S2|pc|S6|pc|DQ|pc|D6|pc|D2|pc|D4|pc|D5|pc|D9|pc|DK|pc|DT|pc|DA|pc|D8|pc|D7|pc|DJ|pc|D3|pc|H6|pc|H7|pc|S7|pc|SJ|pc|SQ|pc|SA|pc|S9|pc|HA|pc|H9|pc|H8|pc|H2|pc|HT|pc|H4|pc|HK|pc|HJ|pc|ST|pc|S3|pc|S5|pc|C5|pc|C8|pc|HQ|pc|S8|pc|CT|]600|400[/HV]

[LBengtsson]

yes, I can explain. west bot does not know who has the ♣9 when east leads to second trick, so overtaking the ♣J with the A will cost a trick if south has this card. so bot blocks suit instead. which proves that if east bot lead ♣9 (4th high) at trick two, it is easiest for human to work south cards and not block suit. whether bot still blocks suit on ♣9 lead also at trick two is a guess...

[pilowsky]

So, in sum, the bot blotted its book by not unblocking?