What does alvarez mean when she uses the word "spark"?

Spark can be used as an a emotion or as describing an electric force.

Hope this helps.

an expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference.

with mrs. linde, nora reveals her insensitivity and lack of shame about the forgery she committed. she implies that mrs. linde has obviously aged since they last met 10 years earlier, which accounts for nora’s inability to recognize her old friend. although nora seems sympathetic to mrs. linde’s plight, she can’t seem to control herself from boasting about her three children, her husband’s recent promotion, and how good she feels to finally have some money to spend. in a way, she seems to be flaunting her good fortune in front of mrs. linde who, deprived of all these benefits, has come to ask nora for . it is also through her interactions with mrs. linde that we realize something deeper lies under nora’s seemingly childish exterior. she reveals the secret of her forgery to mrs. linde and seems to see herself as quite resourceful for having done it.

in the company of dr. rank, nora is freer than she is with her own husband. she openly laughs with joy at knowing that all the other bank employees will be under her husband’s control. she offers dr. rank a macaroon, apparently forgetting that he knows she is forbidden to eat them. she then quickly covers for herself by claiming that mrs. linde brought them. nora seems much more assertive and free with these other characters, as well as unabashedly willing to scheme, lie, and justify her transgressions as altruistic.  

krogstad provides an interesting parallel to nora. like nora, he too is guilty of committing forgery, for which torvald condemns him and decides to fire him. although krogstad is a man and nora a woman, their crimes of forgery unite them. in fact, krogstad might represent what nora would be like if she were a man and were not strictly scrupulous like her husband is. for example, she tells mrs. linde that "a barrister's profession is such an uncertain thing, especially if he won't undertake unsavoury cases; and naturally torvald has never been willing to do that." perhaps nora would have delved into the unsavory, given how opposite she is to her husband. krogstad's is also the only harsh voice of reality that nora is forced to hear, and that is because he blackmails her for her regaining his job at the bank. finally, torvald’s reaction to krogstad’s forgery seems to foreshadow his harsh and cold reaction to nora’s forgery.