Let's make the first thing very clear, we are referring to the conventional deadlift, not the sumo deadlift. Both are entirely different beasts.
One of the main technical faults we see when first assessing new lifters is the incorrect foot position, mainly where they have their feet too wide. An easy coaching correction to make which results in an instant technical benefit.
Before we get in to details let us remember that lifting heavy weight is about using our bodies the way they were intended and then trying to squeeze every ounce of mechanical leverage out of it so we can move more weight!
Feet out wide mean our hands have to be further along the bar which has a massive impact on mechanics because we have to place ourselves close to the bar with a more acute hip angle. A significant mechanical disadvantage.
Have you ever performed a snatch grip deadlift with hands out wide? Hard right? Of course! The more acute the joints are the more difficult force production is in that range and the more range we have to move the bar, so more work. Exactly the opposite of what we are trying to achieve.
To be honest we don't see this a lot, at all. But it's worth mentioning. It's an issue because your arms tend to get in the way of the legs and you'll find the knees cave in as a result really minimising effective hip stability and therefore the ability to transfer force.
Now, before we get to the perfect foot position, try this at home... a vertical jump. Do one with feet out wide and one with feet touching. Then do one with the feet directly under the hips. Tell us which one result in a higher vertical jump?
You guessed it? The foot position directly under the hips (coxal joint) has the higher vertical jump. But how does this translate to the deadlift? Great questions. Well, this foot position means you are in a more effective position to generate force and see that force through to the bar. More force generated results in more weight being able to be moved!
Sure it's not always that simple and we are all different but something to consider.