A follow on from my post about the true feel and look of connectedness.
The curve under the neck is another huge tell-tale as to whether a horse is truly working correctly.
The curve between jaw and chest should be upwards, following the same line as the curve at the top of the neck between poll and withers. The under-neck muscles should be soft, allowing this curve.
If it does not look like this, if it is actually a downwards curve from jaw to chest, there is no true give at poll and/or jaw, there is tension and resistance and force being used to counter these deficits, and to make the horse arch its neck and bring its face to the vertical.
A horse which is genuinely through at poll and jaw will show a relaxed underneck.