LEARNING CENTER

NF. A negative double followed by a new suit is weak (the opposite of a takeout double followed by a new suit). Here, if responder had a strong hand, he would have bid 3 the first time. South likely has something like: 

 A1032  
 2  
 32  
 QJ10876.
 

NF. To force, opener could cuebid (2). This is not the same as opener's jumpshift with no interference. South shows something like: 

 32  
 AQJ87  
 2  
 AQJ106.
 

NF. Same concept as the previous example. South's jump is extras/invitational, but a cuebid (3) was available to force.

NF. This could be a dead minimum. What else could opener bid with, say, 

 43  
 AQJ76  
 72  
 KQ32?
 

NF. This shows a huge hand, but to force, South could have cuebid (2).  North can pass if totally broke.