Source: p 105, The Law of Contract, 5 ed (2012), by O’Sullivan and Hilliard
It is encapsulated in the difficult seventeenth-century language of Lampleigh v Braithwait (1615):
A mere voluntary courtesie will not have a consideration to uphold an assumpsit. But if that courtesie were moved by a suit or request of the party that gives the assumpsit, it will bind,
for the promise, though it follows, yet it is not naked,
but couples itself with the suit before  and the merits of the party procured by that suit,
which is the difference.
This means, if A asks B to do something for him and later promises to pay for it or do something in return, A’s promise will be enforceable because it is supported by consideration, namely what B did at A’s request.
1. Am I right in guessing that Definition 2.1 applies? But how does it make sense for a suit to procure 'merits of the party'? What did I misinterpret?
2.1 = (archaic or Law) Cause (something) to happen:
2. What's the antecedent of which? Which 'difference' does it concern?
Is it whether a 'courtesie' has consideration or not?