she sells seashells type of rhyming style

I'm not quite sure what the "specific type of technique" is that you're asking about. The specific name for the technique of matching vowel sounds (or consonant sounds) without a typical rhyme is assonance:

Resemblance of sound between syllables of nearby words, arising particularly from the rhyming of two or more stressed vowels, but not consonants (e.g. sonnet, porridge), but also from the use of identical consonants with different vowels (e.g. killed, cold, culled).

— Oxford

It's a tongue-twister.

A tongue-twister is a phrase that is designed to be difficult to articulate properly, and can be used as a type of spoken (or sung) word game. Some tongue-twisters produce results that are humorous (or humorously vulgar) when they are mispronounced, while others simply rely on the confusion and mistakes of the speaker for their amusement value.

The she sells sea-shells one uses the technique of volatile changes between the [s] (ss) and [ʃ] (shh) sounds, which result in very difficult pronunciation.

Another example of a [s] tongue-twister is

The seething sea ceaseth and thus the seething sea sufficeth us.

The techniques used in tongue-twisters include sibilance, where s-sounds are repeated ─ present in she sells sea-shells.

Information on Wikipedia

Source: Wikipedia.