In securities trading, we have a particular term which I think has broader applicability, so I'm seeking a more common (or at least more universally comprehensible) term for it, that I can use in casual conversation (outside the industry).
The term is penny-jumping, and applied literally, means "taking advantage of the knowledge that someone else's order will move the market, by placing a similar order at a trivially improved price ahead of his", thus "riding his wave" to your benefit and his expense.
But we apply it more liberally than that: it essentially means a kind of obnoxious parasitism¹, and apply it to a variety of situations.
More common example:
Imagine standing on the curb for 15 minutes trying to hail a cab, only to have another person jump in front of you (by a few feet or a single block), and flag "your" cab down at the last minute. While you've waited for 15 minutes, they've only had to wait a few seconds, and they were able to benefit (to your detriment) by making only a trivial "improvement" over your "bid" (those few feet they stepped out in front of you).
Of course, the analogy isn't perfect, but analogies rarely are, and that's how we use it.
Another term of art for this, in the industry, is front-running (though the nuances are different and front-running more explicitly refers to the illegal practice) , though I think penny-jumping captures the same idea more descriptively: the parasite jumps ahead of you for a cost which is trivial to him (a penny), thereby capturing an outsized reward at your expense.
¹ If done using non-public information, particularly information derived from a privileged relationship with a client (to whom you owe a fiduciary duty), penny-jumping is illegal and is usually termed "front-running" (which is the name of the crime, historically committed by brokers who intermediate between clients and markets). But I'm not concerned with this sense of the word; I'm seeking something which is sleazy and obnoxious, not illegal or fraudulent: think "parasite", not "criminal".