The classic form is a broker who receives a large customer buy order and buys for the firm or a personal account first. Variants include trading ahead of a research publication, ahead of an index rebalance the firm arranged, or ahead of a block the desk is shopping.
The duty breached is to the client, so this is charged as fraud and as a violation of firm conduct rules. Surveillance now reconstructs sequences from the consolidated-audit-trail, which makes the timing evidence unusually clean.
In crypto the analogous behaviour by block builders and searchers is often described as sandwiching and sits largely outside securities law, which is a difference in regulatory reach rather than in economics.
Related: best-execution, consolidated-audit-trail, insider-trading, churning, chief-compliance-officer