What is the benefit of agitating gently, particularly with pushing Kentmere 400?

Question

In this post about pushing Kentmere 400 to 1600, it talks about gentle agitation:

• agitate very gently (like with the little swizzle stick thing from the Patterson tank) for first 30 or 45 seconds, snap on the lid, and tap gently to release air bubbles
• Develop for 20 minutes, with 30 seconds of uber-gentle agitation at 6:30 and 13

Typically my development process is more vigorous - I agitate every 30 seconds or so. So far it works fine, though I haven't pushed Kentmere 400 before.

What is the point of agitating gently? How is it any different, and if there is a difference what happens?

Answer 1

When you push film, i.e. develop for longer than usually recommended for that particular film–developer combination, you increase the contrast index or gamma, resulting in negatives with higher contrast than normal development would yield.

The more you agitate, the more fresh developer is going to replace exhausted developer at the emulsion–developer interface. This also tends to increase contrast.

Thus the recommendation to agitate only slightly when pushing may be intended as a means of controlling contrast. Be careful though – insufficient agitation causes problems too (uneven development). Some experimentation is probably necessary to get the best performance out of any given film–developer–routine combination.