You hit the buy button. The screen flashes. A confirmation appears three seconds later — or doesn't. You check the position tab. Sometimes it's there. Sometimes it's partial. Sometimes you're staring at a rejection notice wondering what the hell just happened. Most traders treat execution like magic — click button, get position. But those milliseconds between your order and the fill contain a cascade of routing decisions, queue priorities, and matching logic that can make or break your edge.
When you submit a market order, you're not buying from "the market" — you're buying from someone else's limit order sitting in the queue. Your broker routes that order through one or more exchanges or dark pools, where matching engines process millions of orders per second. If you're trading ASX stocks, your order hits Chi-X first or ASX directly, depending on your broker's smart order routing. The matching engine pairs buyers and sellers using price-time priority — best price wins, and if prices match, first in line gets filled. That's why you sometimes get partial fills on illiquid stocks. There weren't enough limit orders at your price level to absorb your size.
The realisation hit me after a string of slippage on breakout entries. I was using market orders on momentum plays, expecting instant fills at the breakout price. Instead, I'd get filled 10-15 cents higher on fast moves. The matching engine was consuming limit orders in sequence — by the time my order arrived, the cheap sellers were gone. I switched to limit orders just above the breakout level and suddenly my entries tightened up. Same setups. Different execution logic.
The fix is understanding what you're actually asking the system to do. A market order says "fill me now at whatever price is available" — you're crossing the spread and paying for immediacy. A limit order says "I'll wait for my price" — you join the queue and hope someone crosses to you. On fast-moving stocks, market orders get you in but cost you slippage. On slower stocks, limit orders save you the spread but risk missing the move entirely. Your broker's smart order routing tries to optimise this across venues, but it's working with the order type you gave it. If your edge depends on tight entries, test limit orders at your trigger price plus one tick. If your edge depends on participation, accept the slippage cost and use market orders. The execution method is part of the system — not an afterthought.
This content is educational only and does not constitute financial advice. Past performance is not indicative of future results. Always seek licensed financial advice before trading.