The sombrero effect refers to ensuring that the instantaneous load placed on a service is maintained at a level no greater than the maximum load the service can support. In the context BoJo used it was in a way usually encountered with telecoms dynamic planning but equally applicable to most systems with a varying load. The ONLY thing he left out was that the capacity of the system, in this case the NHS and ancillary support, is itself affected by the load being applied.
The "sombrero" relates to the curve / time and limiting the hight of the sombrero to go no higher than the system capacity ie squashing the sombrero means that while the area under the curve of the squashed sombrero might be somewhat greater than the sombrero, by remaining within the capacity of the system capacity the optimum result will be achieved.