The pierce relationship requires two vectors, it could be established between point to vector,vector to point.
It’s important to keep in mind that pierce relationship requires two separate sketches.
1.Point to Vector:
Let’s make a circle on the top plane for example,and a line on the front plane,as shown in the fig below.(fig.1)
While staying in the sketch mode;select the end point of the line that you wish to pierce on the circle then ctrl hold and select the center point of the circle.
A pierce relation option will appear in the Add Relations box at the bottom of the left side menu.(fig.2)
Clicking on the Pierce relation will drag the line to be pierced at the edge of the circle as shown in (fig.3)
Notice that the line is now fully defined.
Now we can make a sweep feature choosing the first sketch(the circle)as the profile sketch,and the second sketch(the line)as the path of the sweep as shown in (fig.4)
The result of the sweep feature is as shown in (fig.5)
2.Vector to point:
In this method the vector is dragged to the point to be pierced at it.
So,let’s say we draw a rectangle on the top plane,and a spline on the front plane as shown in (fig.6)
selecting a vector point on the spline, hold ctrl and select one of the rectangle’s edges,then select the pierce relation from the Add Relations box.The result will be as shown in (fig.7)
The result is as shown in (fig.8)