To those well versed and practiced in the Yang Long Form, the sequence that begins the form and reoccurs another eight times throughout the whole long form:
Grasp the Sparrows Tail - Ward Off Left
Grasp the Sparrows Tail - Ward Off Right
Grasp the Sparrows Tail - Rollback
Grasp the Sparrows Tail - Press
Grasp the Sparrows Tail - Withdraw
Grasp the Sparrows Tail - Push
and then ... Single Whip...
come to know and refer to that whole sequence as
"Grasp the Sparrows Tail - Short".

This 'short' sequence as described above opens part one two and three. In part one this is followed by 'Lift Hands' and 'Shoulder Strike' which points the practitioner towards the east (Dragon). In part two of the Yang Long Form this 'connecting' sequence of (to abbreviate even further) 'GST Short' is followed by the sequence of Fist Under Elbow, which is performed only once within the Yang Long Form.
Grasp the Sparrows Tail Long sequence ends with Wave Hands In Clouds then Side Single Whip. Of the nine performances of this sequence within the three parts of the Long Form; six are 'Short' therefore three 'Long'.

The body that adopts Tai Chi postures should not be considered as separate bits and pieces connected by other disassociated bits and pieces. It should, by the single viewer (the 'mind' of the viewer/the practitioner), be allowed to become just One. Just one droplet of morning dew, or just one vast heaving ocean. No matter which - just One.

Likewise, Tai Chi Form should not be viewed as separate postures held or brought together in some way by other postures that are in any way different from those that they precede or follow. One 'flows from' or simply 'becomes' the other.
Bearing all of this in mind, it can only be said that the sequence ...
Fist Under Elbow'begins' where Single Whip ends.

Release (soften) Single Whip and lift both hands to above the right shoulder, as if holding a ball. The left foot is brought into Cat Stance with all of your weight in/over the right leg.

Click on any figure (pose) below to access link: Fist Under Elbow - Chuan Applications.

Both arms are taken (still holding the ball) to the left hip

The right foot is brought to the front in an arch, toes leading, inwards to out. The hands at the same time (and still holding the ball) describe a semicircle an arms length distance from the hip.

This leads to the singular posture that gives the sequence its name: Fist Under Elbow

Repulse Monkey Three Times sequence begins where Fist Under Elbow ends.