Thanks to a recent question on ELL, I discovered a new meaning of spot. User @John Feltz defined it as:
In a gym, to "spot" for someone means to help them with heavy weights
The Oxford Dictionary Online defines the verb:
1.4 (in weight training, gymnastics, etc.) observe (a performer) in order to minimize the risk of accidents or injuries.
Now, I understand that the verb to spot, meaning to see, pick out, or identify something, is derived from the noun spot, usually a small dark roundish mark or stain. Imagine seeing a ‘spot’ in a distance, if you can identify what it is, your eyesight must be very good. From there the leap to the British English trainspotter is relatively brief. To spot a train didn't mean to "see" a train coming, it meant finding, and collecting steam trains' numbers, and their names. It was a fascinating hobby for British schoolboys and grown-up men during the 1950s and '60s (ahem...)
But in the US, how did spot mean to help someone not hurt him or herself in the gym? I don't spot the connection.